Cut a hole through the upholstery of your backseat about the same size or little smaller in diameter than your cock (you won't want to do this if you have leather seats). Carve a hole into the foam deep enough to accommodate your erect penis. Lay across the backseat and hump the hole. Use a condom or insert a plastic bag into the hole to avoid creating a spunk-filled waste dump in the cushion. Conceal hole with a seat cover when not in use.