Sex in the car is usually bad. Err, well, Dean's always there, so it's not bad, just not the greatest. Sam's legs cramp up, he looses his balance and falls off the seat, only to be stuck on the floor, trapped between the front and the backseat. His favorite is when he hits his head on the roof.
But that's backseat sex. The first time they have sex in the front seat, it's after 2 am and they're on a deserted highway in the forest somewhere near the coast of California - or maybe it's Oregon.. - Sam's not sure, and when Dean wakes him up by crawling in his lap and deftly removing Sam's hoodie, followed by pants, boxers, and boots, then Dean's clothes are gone too and Dean is reaching behind himself and digging in the glove box for lube, Sam starts to think twice about car-sex.
Sam always assumed that front seat sex would be impossible, too many things in the way. But Dean is in his lap with his hand fisting Sam's hair and guiding Sam's cock into his slick hole with the other. His harsh breath is steaming the windows and his moans echoing in the vinyl and leather. Dean rocks slowly, deliberately guiding Sam's cock into him, making sure that with every thrust, Sam is pressed against his prostate, and Dean is coming, yowling Sam's name. Sam's never gotten a better view of Dean's face when an orgasm plows through him. Dean's eyes roll back, his mouth is open so wide that Sam's sure he's going to unhinge his jaw, and Sam's not even going to bitch about Dean pulling his hair so hard that he's sure he's going to find a bald spot in the morning because those hands in his hair, threading and pulling are what drives him into his own orgasm.
His last thought before his body goes limp is, front seat sex is good.