I build all my vehicles the same way as you would build a real vehicle, one component at a time; fenders, doors, hood, etc. I believe this is easier than trying to build the entire vehicles body as one mesh. Difficult surface transitions in sheet metal will probably be equally difficult with patches. And I think that having scale gaps between the parts adds an important degree of realism.
A quick comment on modeling. My style is not "extrude and tweak". I prefer to use rotoscopes as a guide as I lay in single splines that define the edges and overall shape of a part. After this I go in and stitch in patches to fill out the surfaces. The downside of this technique is that you don't immediately start with renderable patches. This may be a problem if you can't look at a a few disconnected splines and imagine how they define a surface.
I'm assuming that you will be using reference pictures of an existing vehicle as rotoscopes. If you're looking for automotive rotoscopes, the three best sites I know of for car drawings are:
If your taking the reference photos yourself, here are a few suggestions.
When you take pictures, make sure the car is the same size in all of the photos. The best way to do this is to use the same focal length lens for all the pictures. That means that if you have a zoom lens, adjust the lens to your preference and then don't change it. Remember you want to use the highest magnification (max. focal length) possible to minimize perspective distortions. Take all the photos with the same zoom setting and distance between you and the vehicle. In those cases where you haven't that option and your various views have different magnifications, you will have to import the pictures into your paint app and scale them accordingly.
Sometimes you may not be able to get far enough away from the vehicle so that you can get a side view with the entire car within the camera frame. In this case you're going to have to build a horizontal mosaic roto in your paint app. Get as far away from the car as you can and turn the camera 90 deg. so its field of view is taller than it is wide. Adjust the zoom so the car's image, from the roof to the bottoms of the tires, just fills the view finder. Now walk along beside the car, from one end to the other, snapping pictures as you go; think of yourself as a human scanner collecting segmented samples of the car. I would take a minimum of six pictures for a typical car. More pictures is better but more work. Make sure that a recognizable landmark appears in each neighbouring picture; for instance, show the gas cap cover in at least two pictures. Alternatively, if you have a long tape measure, lay it down beside the car and make sure you can see the foot markings in each picture. Now import these pictures into your paint app and build up a complete picture of the car by cutting and pasting sections from each image, using the visual landmarks or the tape measure marking to align each section. It won't be pretty but you'll have a useable rotoscope.
In the case of the Chevy pickup, I bought and quickly assembled a 1/24th scale styrene model. Its size made it very easy to get the entire vehicle, including the top view, into the camera frame. I was also able to draw lines on the door gaps to make them more visible.
When it comes to measuring the vehicle, the obvious choices would be length, width and height. But what if you saw the perfect car in a parking lot and brought your camera and forgot to bring your measuring tape? Go ahead and take your reference pictures and when you're done, just pace off the car's wheelbase (the horizontal distance between the axles). This is the best way to measure a car if the owner happens to be sitting in it and he\she looks like the sort who wouldn't let anything sharper than a sponge near the paint job. I'll show how you can deduce the other dimensions from the wheelbase.
Once you've opened an A:M modeling window, the first step is to lay down markers to outline the car's length, width and height. This is easy if you measured these dimensions on the full size vehicle. But let's say that you only have the wheelbase. Go to a side view and set up your marker lines. Put in a horizontal Y marker at Y = 0 to define the ground. Divide the measured wheelbase of the car by two (the pickup was 110"/ 2 = 55") and put a vertical Z marker at +55" and -55".
Bring in the side view image as a new rotoscope and choose the appropriate view in the rotoscopes' general properties window. Grab a corner of the rotoscope and hold down the Shift key so that you can scale the picture while maintaining its proportions. Scale the image so the wheelbase matches the Z markers and position so the wheels sit on the ground marker. Now put in a Y marker that touches the roof.
Switch to a front view, put in a vertical marker at X = 0, scale the view until you can see both Y markers and bring in the front rotoscope. Then scale the image until the wheels and roof touch the two Y markers and the image is cut in half by the X marker. Then add two vertical markers that just touch either side of the picture. Check their values and make sure they are equal but of opposite signs. If not, re-adjust them such that this is the case and move the rotoscope to fit.
Go back to the side view and move the two Z markers to show the front and rear limits of the car.
Switch to the top view and bring in the top rotoscope. Scale the image proportionately so this image fits within the X and Z markers.
Uncheck the Pickable box in all the rotoscopes so you don't accidentally move them. Your rotoscopes are now scaled and positioned so you'll build a correctly scaled, full size car. Whether or not you erase the markers, is up to you.
At this point I should mention that these are the best suggestions I have for making your rotoscopes as useful and accurate as possible. But it's highly unlikely that they will be perfectly aligned and should not be considered the ultimate authority on what looks right in the 3D model. Occasionally you'll lay in a spline in one view to match a part edge, switch to a different view of the same edge, and the spline doesn't quite match. Don't worry, just split the difference. This is not CAD, this is 3D animation. If you think it looks right, it is.
I'll concentrate on the front fenders since in many ways it's the hardest part on the pickup to model. Start with a side view and add a spline that outlines the top of the fender, putting in cps (control points) every 6" to 8". I turn on a 6" grid to give myself a visual reference.
Switch to a top view where this spline shows as a vertical line. Move it so the spline lies along that section of the part that was the template for drawing the spline's profile.
Go back to the side view and build a spline along the lower edge. Do your best to use a similar number of cps as on the top spline i.e. if you've got 12 cp's on the top, don't put 35 cp's on the bottom. You'll notice that I put a few more cp's at the leading and trailing edges since I knew these would have to curve around either end.
Look carefully at the cp locations and decide how you would best connect them to form legal patches. If you can't do it with a 2D shape, you probably won't be able to do it in 3D. Add or remove cp's as required.
Note: Don't obsess on getting all the cp's in exactly the correct location. All you're after is a roughly fender shaped object. The finessing comes after you've got the general shape with minimal creasing.
Switch to the top view and move the lower spline to line up with the outside edge of the fender and bend those extra cp's around the ends. Copy the spline that runs along the top of the fender and line it up with the inside edge.
Hide the rear halves of these three splines and switch to the front view where you can line them up with their respective edges and complete the spline defining the front edges. Repeat the same for the other ends of the splines, switching amongst the three views to get the cps as close to the rotoscopes as possible. Lathe an eight point circle, scale and position it to define the outside of the headlight hole. This doesn't mean that eight pts will be what you'll finally use but it's a safe estimate.
By this point, your splines will start to define the overall shape of the part.
Switch to a side view and hide all but three cp's that are more or less aligned in the Z direction. In a front view draw a spline that mimics the shape of the fender and more or less joins the three visible control points.
Grab this whole spline and move it in Z until it aligns with the three cp's. Unhide the rest of the splines and join them to the ends of the latest spline.
Now you've got the major styling lines of the fender defined. At this point the rotoscopes have served their purpose. Unless you have very detailed section drawings that show all the major contours of the surfaces, your better off using your own best judgment for building the rest.
Stitch the cp's together to form patches across the surface. Unless your very lucky or very skilled, at this point you'll have the ugliest fender you ever saw but at least it's close to the right size and shape. The next part is the real grunt work, tweaking the surfaces for maximum smoothness. From here on in your main tools are patience and perserverance. You'll have to add, move and delete splines to get the best tradeoff between detail and smoothness.
But if you keep at it and use some of the tips I offer in the other tutorials, what you end up with is one decent looking piece of sheet metal. Then you just keep running that play with the rear fenders, hood, doors, cab, etc., until you're finished.
For suggestions on adding bones to your vehicle, try here