Flexibility is key
Unless you or your parents own an Astrovan, let’s hope that everyone involved is either small or flexible.
Expand your space with a sunroof
If you have a sunroof, that can be opened for more headroom -- but you’re still likely to be encumbered by a stick shift or gear handle.
If you have a backseat, use it
You about that sedan life? The backseat makes more sense, and at least one of you can lie flat on your back with your legs raised and spread. The on-top thrustee needs decent amount of upper body strength, since they will be in a half-plank position.
For trucks, utilize the recliner, the height, or the bed
If you’re getting busy inside an F-something quad cab, you might as well pull the passenger seat all the way to the back. You know there's no room in that backseat.
However, if he’s tall and the truck isn’t too lifted, an assisted standing position is totally doable. The person being penetrated can rest most of their weight sideways in a seat, while the penetrator stands outside of the open door. Call it T-boning!
Bed of the truck? Sure, if you’re somewhere remote and you don’t mind bugs biting your ass. AND being on the bottom isn’t comfortable in the corrugated beds... unless you have a few hoodies or blankets to soften the surface beneath you.
Eyes on the road in compact cars
In a small car like my VW, I'll have my lucky passenger sit in that seat, and I will sit on their lap with them inside of me, both of us facing the same direction. He can assist me in my humping and grinding, and if trouble comes, I can quickly scramble back into my driver’s seat.
There’s something charming and tacky about the opportunism of humping in a hooptie. Humans are ingenious apes. We started doin’ it in our rides the second the Model T was hot off the assembly line -- and you can bet, as self-driving cars become the norm, highways will be filled with all kinds of kinky fun happening just behind those tinted windows.