Unfortunately James and I haven’t had a car sex experience. I’m quite sad about that. So I can’t really help you in terms of how to approach it.
My best guess is just to give it a go, you won’t know unless you try!
Cars aren’t that spacious, so you might not be able to get into some positions you can at home!
Possibly have a test run? Try out some positions you think would work in your car, at home!
You can’t fail at having sex, it’s all about trying things out and making it work for you. Don’t stress too much if it doesn’t work out how you wanted, put it down to experience, and next time you try, you might be better equipped.