Glad to share what info I have. However, my knowledge of UV unwrapping specific to a car is limited to what I've picked up looking at other folks' work.
To start understanding what to do when unwrapping a car, import some of ISI's cars. Either the whole car or just the car body. Select the body, tab into Edit Mode, make sure the whole mesh is selected - not just points or edges. Open the UV/Image Edit window to see the UV map displayed. Study the way the ISI folks have created their UV maps. Their approach to UV map layout appears to provide the best environment reflections (if you are adding those to your car).
There are a couple of concepts that are consistent. And a couple of Blender unwrap tools that work with those concepts.
The first concept deals with seams. Seams (much like clothing) are where two areas of the car mesh meet (and separate). The primary tool to use here is Blender's Mark Seam - found in Edit Mode : Mesh : Edges : Mark Seam. The body does not have to be modeled as separate meshes to use seams. The Mark Seam tool allows the UV map to separate at the marked seams without affecting the actual mesh.
The second concept deals with islands. Islands are sub-images that appear on the larger image/texturemap - like headlights or brand logos. To create an island UV map, use either all the faces of a separate mesh or a sub-selection of faces from a larger mesh.
There is more detailed info out on the web than I can provide, but focus on the unwrap info dealing with seams & with islands.