Troubleshooting turn signal and brake light

I've been fighting with my turn signal for a while now. I cleaned a ground, replaced all of the LEDs with incandescent including inside the cluster, and replaced the flasher. Everything is now working except the rear left turn signal and brake light. That bulb is brand new and it lights when I turn the headlamps on, but doesn't flash on turn or light on brake pedal depression. One other thing that I've found, although I don't know that they are at all related, is that my hazard switch doesn't work. All of my other exterior lighting is working. Where are some good places to start to try to finally isolate the problem?

The brake and turn/hazard are the same filament. And hazards tie all 4 turn signals together without fully isolated contacts (as it's normal during hazard use for all turn signals to turn and stay on when applying brakes). Since the issue is just the one filament for everything but hazards, I suspect you have a partial short somewhere between that wire and ground; the lack of isolation with the stock hazard circuit is why that partial short is taking out the hazards but not anything else on the other signals. Gary had a similar issue when one of his LED modules shorted on him. Assuming you've tried different bulbs, then the next suspects would be wiring damage or a short in the bulb socket.

^ What Larry said, and I would be suspicious of the turn signal switch.

They can cause some odd problems.

I found the problem. It looks like it's in the connector that has the built-in 4-wire trailer harness:



Is there a good replacement for this part? I don't need the original connector, just a good 2-in-1 tee like what's already in there? I have WP connectors if that is the best way to do it instead of a specific part.

I can't quite make out which connector has the problem.  But in the MPC I find two harnesses that are associated with trailers.  The first is 15436 and it appears to be the one that comes from under the hood all the way to the back for vehicles with the trailer tow package.

The second is 13A576 and I think it is the part that connects at the back of the truck.  But I find no illustrations for it, just the part numbers below.

Given that, it might be easier to put your own connectors in.

I'm having to move further up the wire to find the failure. I'm looking at my electrical schematics specific to my Bronco. I am seeing a reference to C-306 near the LH apron, but when I look at the EVTM for the 84 Bronco, it does not show that connector anywhere in the diagrams. Is it possible this is actually C-121 and my schematics are inconsistent?

Chad - Yes, I think you've found another inconsistency between the wiring diagrams and the EVTM.

In the '85 EVTM C121 is said to be an 8-pin connector and resides on the "LH Dash Panel".  But it is also said to be shown on Page 14 in Illustration 1, and there it is on the firewall and near the LH apron.