Personally, the best way that I found is this: You (The Guy) sits in the passenger seat. Her (your Wife) sits in your lap, back towards your face. Another way that works well is: Passenger seat goes all the way back, and reclines all the way back. Girl lays down (can either be on her stomach or back) and guy gets on top. Is able to use area where feet normally are (and still are) as leverage.
I like the idea of the doggy style position, never did try that. But when you're 17 and 16 respectively (this was a few years ago) than you don't need to do anything that will attract attention to yourself or car.