Just take your girl to the backseats, move the front seats to as far as you possibly can. I am sure there will be adequate space then you can have normal sex, remember taking your clothes off would be a little awkward but you can do it no problem.
Use the less space to your advantage, your girl would be more close to you so it would easier to do a lot stuff while a lot of stuff also goes unnoticed if you know what i mean 😉 Enjoy
The back seats have a lot of space to do all the stuff you need to do. Just remember to park your car in a area where you are not disturbed and well go on to the job i think you will do wonderfully!