GM-GMC: brake lights, brake light switch, electric brakes

Question
I have a 1998 chev k1500  truck  the brake lights don't work when I apply the brakes I have replaced the stop light switch. I have electric brakes for my trailer and if I push the bar for the trailer brakes the brake lights work. any ideas  

Answer
I suspect that the wiring between the trsiler brake control and the original brake light switch is at fault.

I would trace the wiring from the brake light switch and the trailer brake control and see how they are connected.  I think that you will find a disconnected wire.