Turn signals cause rear brake lights and high mount brake light to flash?

[OrdinaryBoar]

2003 xlt 4.0L Ranger here.

I've spent the last few days trying to fix my turn signals and brake lights which began acting funny out of the blue.

The problem:
Activating the right or left turn signal on the multifunction switch causes the rear taillights and 3rd brake light to all blink.

Depressing the brake pedal when either of the turn signals are running causes the dash arrow to dim slightly and stay illuminated. Additionally, when the brake pedal is depressed with the right or left signal engaged, the taillights will stop blinking and remain steady illuminated. The tail light that is supposed to have the signal flashing will slightly dim until the brake pedal is released.

The front signals and hazards work fine.

I'm not amazing at electrical but I tried the following fixes without success.

1.Replaced multifunction switch
2. Replaced brake switch
3. Checked side fuse box fuses
4. Replaced flasher relay under dash

My taillights and headlights are aftermarket LED but they have been on the truck for a few years now. Before the problem occurred, I had pulled every fuse from the fuse box to check them. The only fuse that needed to be replaced was for the dome light (I believe #11?).

I'm still trying to find a solution and would appreciate any help. My focus today is to see if I can find any issues with the taillight grounding/wires.

Lastly, There is what I believe to be a factory tow package that has a messed up 6 or 7 pin connector. It looks like the previous owner ran a wire from the trailer wire harness for a bed cap brake light. It isn't plugged in, but there is a tangle of connected and unconnected wires that would connect to the pin blades.

If any specific pictures would help, let me know and I can post them.

 

[RobbieD]

Lastly, There is what I believe to be a factory tow package that has a messed up 6 or 7 pin connector. It looks like the previous owner ran a wire from the trailer wire harness for a bed cap brake light. It isn't plugged in, but there is a tangle of connected and unconnected wires that would connect to the pin blades.

That needs to be fixed, before you do anything else.

And once that is fixed, you probably won't have to do anything else.

 

[OrdinaryBoar]

Perfect. I can probably do that. I've seen the terminal to wire color diagrams here, should I follow those and temporarily hook up the terminals? Or would it be better to cap the wire ends?

I never tow anything so seeing this thing has really rubbed me the wrong way.

 

[RobbieD]

If it were me . . .

I'd cap all of the wire ends, eliminating any compromised plastic insulation in the process.

Get the basic lighting problem resolved first.

That trailer connector looks to be well past its "best by" date. Once your lighting problem is corrected, if a trailer connector is wanted, I'd replace that one with a high quality new one.

Good luck!

 

[Curious Hound]

Sounds like a ground problem causing the turn signal filaments to find a ground path through the brake light filaments. As mentioned above, clean up and/or remove that messy trailer plug harness. Re-analyze performance. Then troubleshoot as necessary.

 

[OrdinaryBoar]

Disconnecting and capping the trailer harness wires fixed the issue. Thanks for the help!

Saved me a lot of frustration trying to test wires.