Back seat sex is sex in the back seat of a car. Many positions are possible in the back seat such as:

I. girl on top with front seat forward and her leaning on it on top of him riding him
II. girl on top with legs apart holding on to the hand bars in the car with her back facing him riding him
III. girl on top facing boy in the middle seat for more space

For most enjoyable results
I. park in isolated place
II. tint your windows three times

III. for a cheap alternative put the sun cover on

Anywhere, anytime.