I just got this truck for my wife.
2004 f150 fx4 scab. 5.4 with 130k on the clock. runs super etc..
while driving today a guy pulled up beside and said my brake lights were flickering when i stepped on brake pedal.
got it home and immediately checked it out. this is what i found
In Park. all lights work normally, brakes, turn signals, running lights.
In Drive when brake is applied they flicker and go dim then sort of come back.
In Reverse same issue dim, flicker

what puzzles me is they only act up in gear. any help is appreciated

 

Ok. just went out again.
with the E-brake on in park all lights function as normal
e-brake on in drive lights go crazy when i use turn signal even inside the arrow goes nuts.
i will keep posting back as i check it over i didnt check bulbs due to it working normally in park.

 

Oh boy, she might have to go into the shop.....

Im not sure whats going on. I went out again and carefully went through different phases. Key on in park turn signal works ok...UNTIL i hit the brake pedal then they go nuts and the brake light is also very dim flicker.
Key on in reverse (engine off) no backup lights if i wiggle the selector lever seems to make the lights flicker then all at once they came on bright like they should. then i applied the brake and back to the same flicker dim etc..

I told my wife this might be something that requires shop time. Uggh i hate electrical issues.

 

It's probably a bad ground for the stop lamp circuit. Check ground G303 behind the LH kick anel.

 

you were on the right track. thanks for the schematic i went through them then on a whim i lowered the spare tire to check the connector where the towing lights plug in i cleaned them well and took out the towing connector and connected them directly. still the same.
then i noticed a small connector right above the frame i reached up and it darn near fell out of its socket. i cleaned it well with contact cleaner and plugged it in till i heard the click....viola!! that was it my guess is the previous owner had unplugged it and didnt get it seated the whole way.
shop time averted.
thanks for posting the schematic again.