2004 ford f 350 super duty, 6.0 Liters international diesel, 4door, 4x4, long bed, dual rear wheels. Over 200,000 mileage. I have no power to the trucks brake lights and no power to the tow plugs trailer brake lights, which is the center left and the center right tow plug blade style contacts, while using a 12 volt test light! All fuses are showing good and I replaced the brake switch at the brake pedal, and I also replaced the towing plug with the short wire and connector at rear of truck which is under the rear bumper and the frame mounted tow hitch bar.1) Does anyone have any information or recommendations?? 2) Are there any relays that control the truck brake lights and the trailer brake lights ? 3) If yes where are the brake relays exactly? 4) Have I missed or not done something else to correct the problem? Please help.,,,Allen.

 

did you use the test light at the brake light switch???

if power is there.. the problem is after the switch... check wires under truck

if NOT, between fuse box and brake light switch.

 

Do the turn signals work? 4-way flashers? On both truck and trailer or not? High-mount brake lamp on truck?

If the high mount brake lamp does work, the switch and it's power supply are good and the issue will probably be at or after the multifunction switch.

More details will help isolate the issue and allow us to point you in the most likely direction.

 

I have found and corrected the problem. The 4 way flashers either had private dirt road dust or something like that in the flashers connection or in the switch. I pressed the flashers on and off repeatedly and realized that the left dash turn signal bulb began working again. So I checked all turn signals and brake lights and 4 way flashers and realized that dust or something was in the contact. I'm glad it's all back to normal function. I'm really surprised that it was a quick and simple and correct repair,Thank you for the help and recommendations ,,Allen