Andrew : I make the seat spring base out of a piece of some kind of elastic, here is a pic of it. I have no idea what it's called or where it came from, probably something from a fabric store, I prowl them every now & then they have a lot of very interesting things for a modeler. You just have to learn to ignore the funny looks the clerks give you as you ooooooooohh & ahhh over things!
Then I stick some cotton fibers in it from a Q tip,the upholstry is a Kleenex painted & installed wet so it will stick. The headliner is made of the same thing, I'll probably use Grosgrain ribbon for the door & kick panels.
The seat frame is the aluminum baking pan covered with Kleenex. One nice thing about building junk cars --they don't have to be perfect.
Before anyone asks, the rubber floor mat is the liner of a package of Basic cigarettes, other brands most likely use the same patterned foil tho. It makes great running board covers too.