I've shagged a civvy in a car, we were young, and both living with our parents - I expect many of us have. Got caught one night by a security guard, we were on the edge of a building site. No problem, I explained why we were there and then drove off.
Best position was to fold both seats back as flat as possible, then sit straddling the handbrake and lie back with a foot in each front footwell and your shoulders bridging the gap between the seats, with her on top.
Getting nostalgic now, might have to book a car meet!