Beyond The Exterior
Seating
The last major part of the interior to worry about before you run off on your own to experiment with adding all sorts of finicky detail, is the seating. Seating is important because it is a large detail that is plainly visible from most exterior shots. Maybe even more important is that without it you'd just have to kneel on the floorboards, and cruising wouldn't be as fun. :-)
If you start with a simple cube sized to approximate the cushion of a bench seat, and throw in a few knifecuts so that you can add a bit of cushiony roundness to it, you are well on your way. This initial shape is the most important.
I then use the grid in the top view to add recurring and equidistant knifecuts, meant to mark the edges of each quilted strip. I don't know if there's a better description for them than that, but you'll see what I mean...
If you select alternating strips (so you can do half the strips at once) and then smooth shift them just a little, you get a padded look. Select the other half of the strips and do the same and the end result is evocative of nice old vinyl bench seats. Notice I only padded the top half of the bench, and used a double knifecut around the middle of the bench to create a familiar vinyl seam when extruded.
Using that technique, you can quickly see how easy it is to complete the rest of the seats. The back cushion is done in the same manner except that it is slightly longer (taller) in shape, and I put the seam in a different place. I didn't get fancy and attach them in any manner, my seats usually end up being just two cushions propped together like this.
Of course, to do the front seat, I used the same bench and made separate seat backs in exactly the same manner as all the other cushions. I love car seats because they look so much harder than they are...