Test , or have the hazard switch tested . Will NEED volt/ohmmeter for this NOT a test light. Brake and signal are the same bulb right? The next thing would be to check for broken wires under the truck , check continuity(ohmmeter) of wires from front to back.If you can get your hands on a wiring diagram , I always recommend that.
Check ALL the fuses you can find with a test light on each side of each fuse. CHECK ALL!! A very simple test that dosen't involve any testers. Unplug the brake light switch connector and jumper and look to see if lights are on.