Hey all! I'm hunting down an electrical gremlin and would appreciate some help.



Trouble
My driver side rear tail light/brake light/turn signal/flasher not working. reverse light works. Passenger side works.


this light doesnt work


Other info
Passenger side lights work, all reverse lights work, Doors locks, windows roll down, etc. everything works... EXCEPT the REAR driver's side brakelight/flasher/turnsignal

Troubleshooting steps taken
Swapped rear harness from passenger to driver. That part that plugs into the taillight housing and there's a connecter that feeds through frame. This step should clear the bulb housing as the trouble.
Replaced bulb.
Ran a grounded wire (yellow) directly to frame
With brake pedal pressed the ‘Major’ and ground wires measure 12.2. Same at bulb pin connector.
Unsure what the ‘minor’ should read on the voltmeter?
Passenger side the working side measures 12.5v at the pin connector 'major' wire'
All front lights and blinker lights work

Misc info
I have aftermarket HID lights with DRLs connected to the front blinkers.
Owned truck for ~4 years, never had this trouble.
There is a trailer brake, but I’ve never used it.

Looking for help
What would be the next steps here?
Is there a fuse for only the rear driver's side? Which fuse would cause this?
Anything else I could test with my voltmeter to help isolate the root cause? "major" wire measures 12.2v. Same at bulb pin connector.
Unsure what the ‘minor’ should read on the voltmeter?
Passenger side the working side measures 12.5v at the pin connector 'major' wire'
All front lights and blinker lights work. Although the driver blinker is a hyperflash (flashes really fast...)

 

I assume you mean brake and running light wires. Check for power at the brake light wire with the bulb connected. No power then check the wire coming out of the multi function switch(turn signal) for the same wire. No power then you have a bad multi function switch. Look up the wires here. Having a set of insulation piercing clips help a lot for this kind of testing.

 

@Moparite thanks for your help! I ended up getting some piercing testers and trying to run cot tests, but struggling because my switch wires dont seem to match the "2001 manual" (there's no wire #17 in mine) - My derdge is a 2002 ram 2500 so it's that weird year between 2nd and 3rd gen.

After messing with it a bit and not getting anywhere, I plan to buy a new turn signal multi-function switch and see if that fixes it. If not, then I'll have to dig into the wiring diagrams and see if I have a bad wire somewhere.

 

I replaced the multi function switch(turn signal). Still the same issue 🥲

Confirmed that I have no power to the (driver’s) brake light when the bulb is installed.

Sounds like i have a bad wire somewhere. This thing is kicking my butt man!

 

Did you confirm you have power out of the MFS? If so work you way back to the light. I believe there is a connector by the spare tire you may want to check also.

 

Do you have a trailer harness? Those are frequently responsible for some odd problems.

 

figured it out boys. finally traced the brake light wire into an absolute RAT NEST of wires comprising the trailer brake harness. Found a bad connection at one of those stupid butt-joint-connector things... soldered the wires together and we're good to go

thanks everyone for the help!