I Have Had Sex In a Car

I suspect this was much more common in the 60's when very few of us unmarried teens had anywhere else to go. It was never the most comfortable experience. In those days, car seats were fairly static. They would slide back and forth to accommodate taller or shorter drivers, but in most cases, the backrests were fixed in the upright position.
My wife (who was my girlfriend at the time) and I managed it several times. Her dad used to lend us his car. His car was huge with bench seats front and back so quite a luxury. There were several places where courting couples would park up for "a bit of sexual fun". I was only home at weekends, so most Saturdays, we'd end the evening there to gain most benefit of the car before I returned both car and daughter to their owner - each with a little more mileage!
I eventually got my own car which was much smaller. We managed it a few times though. The back was very cramped and out of the question, but with the front seat pushed back as far as it would go, me starting off kneeling in the footwell between her legs, and hands on the back of her seat, we used to manage it. Condoms were essential in that car - there was no guarantee of a quick withdrawal without serious injury.