Hey Scofield.
Franco, Escher and Zphix have all suggested terrific things. Believe them, it works.
More often I have done this with young women that I have known for more than 72 hours but it still works all the same with women you've just met (if they are willing to get in the car with you that is).
A few things to remember...
Always assume attraction because, hell, she's in the car.
It's good to find a discrete location to park. Women love fooling around in public places, which is not the same as PDA. Keeping it discrete is very important to them. I use places like, high school/public baseball, football, soccer fields, parks and recreational areas are great. Or the good-old parking lot will do, however not that exciting and best used at night time, somewhere not too dangerous.
Music, anything really works, maybe not classical, haven't tried that yet.
You driver's seat, her passenger seat.
If for some reason you both still have your seat belts on, then, while your talking, slowly take your seat belt off, but don't acknowledge it. You can use this as sort of a test to elicit an ioi.
If she doesn't take hers off you can tease her about it, while giving her a confused look while moving your eyes down her chest to the seat belt say something like, "I'm not driving that fast am I?" or "Don't worry, my airbags are top-notch, five start rating." or depending on the vibe, "I've heard about safe-sex but is that really necessary?! (gesturing at the seat belt)", remember, always followed by a sexy smile.
If, however, she takes off her seat belt after you do it (the sooner the better), its go time. She knows what's up.
Kino is oh so important here. Her legs, shoulders, head, hair, hands, arms, hips, bum, everything other then her feet (unless she's tiny and sits on the seat with her legs up) are mere inches away from you.
Rough guideline, using your hands, the succession of Kino I use is as follows: hand to hand, hand to arm, arm then to shoulders, shoulders then to hip, hip then to leg, hand to face, face to mouth, mouth to mouth. Eye Contact Flirting is a must during this process.
When the time is right, Escalation Window her I come. Move in for the kiss. Push-Pull. Escalate. Deal with her uncertainty accordingly.
Assuming everything is going well and your tongue is in the pretty girls mouth, you should go to the back seat when things in the front start to get hot and heavy, almost too much for the front seats. You both need more room. From here on its up to you to lead her, that's all that is required.
Moving to the back seat...
With the sexual tension built up, through teasing, touching and good make-out/foolin' around skills, the time has come, you simply tell her to get in the back. This is an expression of dominance.
Other times, if I wanted to escalate, I would pull away from the intense make-out session and jump in the back, before she could react, and say something like, "Get over here."
This is how I do it. All vehicles are pretty much the same, interior wise. Unless you have a van and you go in through side doors and are already in the back seat, you'll want to use a strategy, like the ones provided here, to transition a women to the back.
Also, Escher's point on taking the interaction outside is a fantastic idea. Having some beach towels and blankets and pillows in your trunk can come in very handy. -dcl