I think this is hilarious, b/c the question @ hand still isn't being answered. Disasterlady, I would say that you might want to start another thread about openings in the truck, as this one is pretty much gone out the window.

My thoughts are they could get in pretty much anywhere, but the most logical approach is that they must be climbing into the vehicle from the ground, and the only parts of the vehicle touching the ground are the tires. If it were me, I'd start my deterrents @ the tires.

To get them out of the truck, also if it were me, I'd pull out the back seats and the carpet, then buy some black cats (the firework variety), and set them off. It is well documented that mice don't like loud noises. You might even be able to tell where they're leaving from, and find the path they're using to help you determine how they're getting in. Do it in the middle of the night over a few nights, they'll get the hint.