As cool as it would be to keep a pet opossum, I'm not sure it would be a good idea to try to take in a wild one because of those teeth. Those little critters have a WICKED set of teeth, and one bite could cause a lot of stitches. (So I would definitely think twice before "grabbing" an adult, for sure.)
If you're not planning on using the car then hey let them stay.
But it really sounds like they plan on staying for quite awhile. Normally when they're startled out of somewhere, they stay away. Seems like they really like your car. If you want it back, you'll probably have to do as starleomach suggested and get a live animal trap, then release them somewhere safe away from people. If not, they're just going to stay and clog up your engine with nesting materials. I would definitely take them more than a mile away, if possible--that close, they could be back.
I have to admit, they sure are cute though. We had an adolescent one coming here once that would come eat leftover cat food on our porch--would actually climb the steps and cross the porch, then give us a "what? I'm not doing anything" look if we turned the light on to look at him (kind of like my hedgehog does if I walk in on him wheeling :cute: )
Good luck figuring out what to do with them!