Odd rear light issue
 

Hi all!
I've got an issue i'd like to hear your ideas and opinions on.
Vehicle: 1989 F-150 XLT, 2WD, 5.8L Stock.
Issue: Right rear light does not work in Hazards mode, or in Brake Light mode. All other lights work correctly, and right rear light does work in Right turn mode, and/or in marker light mode.
Done so far: I've checked all the fuses, all good. I checked, and swapped, both flashers, no change in issue. Returned flashers to original position.
Checked wiring in sterring column by taking wheel off and visually and electrically checking wires, no problem found, all operations normal.
I checked the light socket, and it's good. It's old and dirty, as expected, but both filaments work without hesitation (in turn signal and marker light modes).
Thoughts? Ideas? Suggestions?

Every time I've had weird, hard to figure out electrical issues it's ALWAYS been a bad ground connection somewhere. I don't know specifically where to look for it in your case, but that's what I'd be looking for.

Could be in the turn signal switch. Id try to find a wiring diagram then use a voltmeter or test light to troubleshoot.

Little by little
 

Thanks for the ideas!
Right now i'm trying to figure out what ties the brake light and the hazards together, but does not include the turn signal.
I know both filaments have power and ground because the turn signal and the marker light work correctly.
The right rear light did have a ground issue when I got the truck, but I fixed that by adding a ground pigtail and a metal screw into the bed inside the light fixture cavity. I checked that again and found that I have no ground issue.

Sounds like bad contacts in the turn signal switch. Check any diagram for the switch; it'll make sense.

Hey guys, thanks for chiming in.
I took it all apart again today, and found that I get power out of the turn signal switch without any issue.
I get power all the way to the right rear lamp (the one giving me trouble) for parking lamps and right turn signal, but not for the brake lamp or the hazards.
I'm planning to take the bed off in the near future to work on the gas tanks, and i'll trace out the wires for the rear lamps when I do that.

Did you check for power at the terminal to that filament at the turn signal switch with the switch in the neutral position, and pressing the brake pedal? Did you check for power at that terminal with the hazard switch activated?

Did you check for _continuity_ between the relevant terminals on the turn signal switch, in the relevant switch positions?

I think I understand what you're asking, and yes, I checked power at the light socket for each condition.
I get power, and light, for the parking light and for the turn signal, but not for the hazard or brake light.

Check for continuity at those terminals when those contacts should be closed. If no continuity, it's the switch.

The saga continues...
So, I have a trailer brake controller installed on this truck, and I wired it to the light green wire in the turn signal harness per the instructions, and it worked fine for a long time.
About three weeks ago when this all started, the turn signal cam broke, so I had to hold the lever to make a left turn. I replaced the turn signal cam with a dorman help part turn signal cam, and the turn signal worked fine again.
Shortly after this, I noticed that the trailer brake controller would light up (indicating it was applying the trailer brakes as though I were pressing the brake pedal) when I was using the turn signals or the hazards. This quickly became a problem while driving!
The issues i've mention above with the lights on the truck transfer to the trailer as well.
So, here's my idea:
I'm going to plug in a new turn signal switch to the connector under the dash (not install it, just unplug the old one, plug in the new one) and see what happens when I activate the hazards, turn signals, and brake lights. If the new turn signal switch corrects those issues, then i'll move forward with replacing the switch in the column.
Here are my two questions:
1) Is there anything you've seen that I overlooked, anything I should check that I haven't already?
2) Why do I have to connect the trailer brake controller signal wire to the light green wire instead of the brake wire?!

Fixed it!
 

Well, it's fixed.
I had several issues, and my instinct (and some of you!) said it was the turn signal switch, so I replaced it, and now everything works correctly.
If you're not familiar with the process to replace the turn signal switch, i've outlined it below.
I also connected the trailer brake controller signal wire to the backside of the green wire on the turn signal harness, just so I can leave it in the truck if I ever have to replace that switch again.


Replacement procedure:
MAKE A DIAGRAM THAT SHOWS WHICH COLOR WIRE GOES IN WHICH SOCKET FOR THE NEW CONNECTOR. Might be a good idea to do this for the old one, too.
Remove steering wheel
Take fuse panel cover off
Disconnect turn signal switch below column
Remove each wire from the plastic connector by pressing the small tab holding the terminals down, and gently pulling out the terminal.
Remove the rubber sleeve covering the wires going through the column.
Tie a string (two to four feet is fine) to the wires, and secure it.
Remove the two screws (silver in mine) holding the turn signal switch in place.
Pull the switch assembly towards you, and work the wire out a bit.
Gently work the wire assembly up through the column while pulling the switch out, and guiding the whole thing through from the bottom.
Reverse procedure to put the whole thing back together.