My left one is not working and the brake dosn,t work either, just the upper light. But it does light up when I turn on the lights. Its not the bulb or fuse. Any suggestions?

 

I think it is the bulb, because it has two different "brightness wires" (sorry about the technical lingo ) The brighter strand probably burnt and broke.

 

If a bulb doesn't fix it, it could be the left brake/signal wire, anywhere from the turn signal switch to the lamp itself. My guess is corrosion under the rear bumper area, usually where trailer wiring is spliced in. Could also be a bad splice in the harness along the left frame rail if someone has added a fifth wheel connector in the box.
May also be a bad multifunction (turn signal) switch, but wiring is the most likely.
Turn on your left signal and check power at the socket first, then continue upstream until you find power. Don't pierce the wire's insulation or it will corrode there. Unplug connectors and check for power at the appropriate pin.

 

I bet it is the bulb. It usually has two filaments, the "brightness wires" that Crimegog is referring to. The dual filament bulb has one filament for dim, and one for bright. Parking and running lights use dim, turn signal and brake use the bright. I bet if you pulled the bulb you would see a burnt filament. Good luck.

 

Geeze guys I said it wasnt the bulb , the reason for this? I already replaced it Anyways it does have trailer wiring in there. I will have to trace the wires and check it out.

 

If it lights but doesn't blink check the front bulb. Also check for a good ground, grounds can cause all sorts of problems.

 

my truck came with the trailering package, i bought it used and the wires were all screwed up...i tried fixing it myself and crossed a couple of wires i shouldn't have. i ended up blowing one of the fuses underneath the hood, have you checked those or just the fuse box in the dash? my symptoms were: no running lights, no dash lights, no blinkers, but i did have brake lights. just my .02