Both of them are out... What fuse should I check? Where do the wires run to? and what else could I look at? 1990 chevrolet single cab 4x4 K1500. thanks in advance.  

the chevy p-up tail lights are a cicuit board type that will crack with time,cold and hot weather. replace both of the taillamp boards and replace all bulbs  

do your blinkers work???
if they do then it's not your brake lights... it's your brake switch... it's a fairly simple wiring setup... and it's the same filiment in the bulb that runs brake lights and blinkers... i think... some of those years had one bulb for brake... one for blinker... so just use process of elimination...  

yeah the board is probably the problem, when the wet weather comes they quit working, I've changed a few of them on my 98, you can find replacement tail light lenses also a walmart for about a hundred bucks for a pair, the boards, if I remember right are 15-30 bucks and just screw in the old lenses.  

I have a 2001 s10 pickup and my tail lites failed inspection because they wouldnt work. i replaced all the bulds and nothing work. maybe its just me, but i found out by accident that it was just a switch that you hav to press a few times. its an overide for daytime running lits. i dont know if your k1500 has this feature but it took me awile to figure mine out.