well here are my thoughts
i have had car sex A TON cause i am only 18, and had a car two years and obviously a high schooler. i do alot of high school game and learn alotta things on my own that other guys cant teach as well (im sure mPuas can teach this, i taught myself no offense to them) so, car sex is kinda what im about
now depends on the situation really, if you JUST meet a girl and wanna fuck in the car, it is easy things happen naturally. you make out some were go to your car, if she hopes in the front turn on some music, kiss then say "up front sucks for kissing lets get some room in the back" and if its a heated up situation, its just simple
OR
if you are just hangin out with a girl and want things to happen in the back, it works along the same lines just a little different. if you havnt built up kino (up to a kiss, after that it tends to be the first catagory) and your going from scratch not gonna lie it can be awkward. the basic way is simple just to get your car into a secluded spot and talk. just tell her your bored of driving and just having one of those days to just kick it and talk. you guys talk for a while, you build up the comfort and kino gradually, play music and stuff make her sing what ever you want. then when things are heated up you need to take it to the back, i do it simple i say "gahh its no fun messing with you up here lets chill in the back we can play a game" now, no need for a game, just do the same kino as before. i tend to just tickle a girl sometimes when she goes back just for fun its so funny at times. once you are back there you can get close, mess with the music and just take it from there. kissing and stuff. gradually if she is liking it and your getting signals to keep going you say "ok now this is pretty adventures are you doing this for your own benift?" she will say "what???", and you say "taking me out here just to have adventures hot sex in a car?" laugh, she will laugh too, keep kissing and say "damn wouldnt that be some adventure, know one would know but me...you...and my tiny ass car " laguh some more. keep building it up a little like that, by keeping the sex in a car in the convo. KEEP IT IN CONVO. from there, things will naturally happen once you mention it she will tend to go for it with some kino and for play, or she might not, sadly though it is hard to do sometimes but iv worked around situations before, and no speicifc routines its always sponatanious things. just work with it
another one is if its late at night and you get to a spot say your tired and say "damnn these seats dont go down..im the cuddle king and i cant even cuddle in a car with you? lets go in the back and cuddle and chill it will be relaxing"
as you can tell from that one, its along the same lines just a lot easier way to say it, and that line to get in the back was my own iv used many times its bullet proof to get in the back

cuddle king
mR.e