Islington,

Help out with a few details:

What are the sight lines like from your theater? What you can/should do differs for proscenium, thrust, or arena seating. (I'll assume proscenium, with gradually rising seats.)

Your basic rule of thumb is first to build the substructure solid and maneuverable, and make everything else as light as possible.

The substructure is where all the blocking and choreography touches the set piece. For the substructure, look at all the parts of the car that take pressure from bodies. If someone leans of part of the car, that needs some reinforcement. If they sit or stand on it, it needs more. If they  jump or dance on it, it needs even more.

Paper mache over chicken wire could work out really good, especially if you spend some time on it (and tin foil "chrome," if you're careful). I wouldn't use minivan seats, mainly because they're too heavy. They only need to look like seats in the sight line.

You'll need something heftier than fishing line to move your car, especially if people are riding in it. Instead of pulling it from the outside, you might want to consider pushing it from the inside. Once again, if your sight lines allow, one or two techs can be in the hood/trunk regions of the car, and enough of your platform cut away to allow them to push the car. You can have a screen in the front of the car that allows you to see out, and the audience not see in.

-Tom