1974 Chevy Truck TAIL WONT COME ON WITH SWICTH

Hello, my wiring info for your year isn't the best. The tail-lites and brake lites don't use the same bulb filament. I'm not sure which one you want to check? Both? Probably a double-filament bulb. Usually the turn signal and brake lite use the same bulb filament, just not at the same time. Check the wires at the bulb socket, brown is for the tail lamp, black is for ground, the other color wire is for brake/turn.