Ok, I have this 78 F150 4wd reg cab flat bed. it had trailer lights wired in when I got it but they didn't work, the wiring harness was one giant cluster F. I bought new LED stop/tail/turn lights and I've been doing a lot of work to the truck. Finally got the lights wired up to the original factory harness, verified that the wires were connect using the wiring diagram (and with some help from other forum members). So tonight I got the grounds in place, and this is what happens: parking lights are great. brake lights, nothing at all. turn signals, the right hand turn signal indicator on the dash is always on. Flipping the lever causes the correct sign to increase in brightness, but not flash. From what I've read, the turn signal thing probably means the flasher needs replaced. For the brake lights, I don't have any idea. Where is the switch that activates the brakes? I replaced all the brake lines, so I don't think its pressure activated like it is on some cars, so I'm guessing its mechanically activated somewhere under the dash. I'll have to check it there. Does it go through any relays after that? I'll have to check the fuse box as well. with a mess of wiring like this thing had it very well could just be shorted out.

 

The brake light switch is under the dash on the upper part of the brake pedal

 

found brake switch. have not tested with multimeter but will tomorrow. What fuse are the brake lights on? I don't have the owners manual and I don't see any of the fuses marked anything like brake lights... are they are on their own circuit? Also from looking under the dash, it appears to me that the flashers are wired one relay/flasher per each side. is this right? The only other vehicle I replaced flashers on had one big one that did both sides.

 

Looking at the fuse panel the fuse for the brake lights is fuse "F8" that's the fuse thats in the lower right hand corner. It should be 20 amps. Thats also the fuse for the emergency flashers. Just so you know, the fuse directly above it, "F9", is for the turn signals, it should be 15 amp.
There are two flasher units, one for turn signals and one for emergency flashers.

 

I took the wiring diagram with me to bed last night and stayed up late reading it with the night light. Lol. I saw that you are correct, one flasher for the turns, one for the hazards, and the hazards and brake lights are wired together. So if it ever stops raining I am going to try the hazards to see if they work, should help me narrow down the problem before I start testing voltages. I have a pretty good idea of how the circuit functions now. I never thought it would be so complicated, but I wasn't thinking about how the same lights are the turns and the stops, so of course it must have a controller to allow that (like a 4 way flat trailer wiring box) built into the turn signal switch.

I'm also going to pick up a replacement heavy duty turn signal flasher while I'm out today.

 

Did some testing, had no hazards either; the fuse labeled "ENERG." was blown and provided power to the hazards and from there to the brake light switch. My truck's fuse block is laid out differently and has different labels than the one in my book. My block also has some others not in the book, one says "L/T" and another says "TPS", not sure what they do. Lol. So replacing that fuse restored power to the switch and now I have brake lights.

Also tested the hazard flasher, it was a heavy duty electronic one and it was not functioning correctly. Replaced it with a new electronic one (LED tails, so the reg ones flash too fast) and the tail lights are working.

Very happy it was such an easy and cheap fix. Truck is one step closer to being "back on the road," though it won't ever really be a daily driver.