I posted a search for this earlier this year but no one was able to find it.

It's set in the bunker, and Sam and Dean's relationship is strained for reasons I can't remember. Sam takes one of the cars out to run some errands but he has to cut his outing short when the car stalls and then catches on fire (and might actually have exploded). He walks back to the bunker to find Dean jerking off in Dean's room, murmuring Sam's name. Cue bottom!Dean.