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Just thought. I would reach out. Maybe get a quick response as I was hoping to go to the airport tonight. Taillights are not working in the 1975 f250. Brake lights work, headlights work, front side markers and turn signals front and rear work. Taillights, rear side markers and reverse lights do not. Horn works. Any ideas? Thanks much everyone. Just was hoping to pick up my daughter in the new truck.
Last edited by Bubba5320; Feb 12, 2026 at 09:33 PM.
Been a real long time since I looked at one, but there should be a connector under the hood going back to the rear that has the taillight loom in it
Look for a break in the wires there, or something unplugged on the way back to the rear (inspect that loom all the way back to the taillights)
If the front marker lamps work, I would assume the switch works, but there are several wires at the switch that might need to be looked at
The rear of the truck where any trailer connector was ever attached is what I would check first
Some of those aftermarket trailer adapter plugs flat out fail and do exactly what yours is doing
Some of them can just be unplugged and the harness plugged back in
Do that if you can and retest
Fun stuff, good luick
Seems to me I saw a plug in a harness wrap there following the gap between the galvanized inner fender and exterior of the cab floor, beside the cab mount.
Also check your grounds (should be inside taillights are) - both at the body and at the sockets. These trucks are notorious for bad grounds causing odd electrical issues. After the last round of cold temps, my left dash indicator lamp would stay on and the signals flashed very slowly - a little cleaning got everything working. Old cars tend to "grow" electrical issues. My other old car needs its hazards cycled several times for the turn signals to come on because the switch contacts turn iffy if it sits.
Thanks y’all for the input. Well, 2 steps forward two step back. Connector coming out of the firewall heads to the rear of the truck square one. Took that apart with some silver grease and put it back together all the rear lights work but now only one brake light works. Right rear brake light and left rear turn signal don’t work. Front left running light and turn signal do not work now. Reverse lamps and license plate lamp don’t work. I think I’ll check the aftermarket trailer hook up as mentioned and see if that solves the brake lights. Turn signal switch, and right rear taillight socket has been replaced. I’m about running out of patience. Lol.
Well. Could not help myself and went back out to the truck. After jiggling that connector that comes out of the firewall and goes to the rear of the truck I get all the blinkers and brake lights to work. so there’s something up with the wires or that connector. Still need to figure out the reverse lights, license plate light and left front running light and blinker. Although the blinker worked earlier today. Obviously some bad connections, bad wires or something.
Last edited by Bubba5320; Feb 13, 2026 at 10:52 PM.
Well. Could not help myself and went back out to the truck. After jiggling that connector that comes out of the firewall and goes to the rear of the truck I get all the blinkers and brake lights to work. so there’s something up with the wires or that connector. Still need to figure out the reverse lights, license plate light and left front running light and blinker. Although the blinker worked earlier today. Obviously some bad connections, bad wires or something.
Was it where I circled above? I still not sure was there?
The connector that I have been playing with is in the engine compartment. Wires come out of the firewall driver side, into a square connector. About 1 in.² and 3 inches long. it connects to another half similar and goes down under the trunk. Does this make sense? I just saw the connector was open a little bit and I jiggled it and the lights came on. After pressing it together and checking it a few hours later, that’s when I found that only one brake light and one blinker was working. I jiggled the wires again and everything worked. So somethings not right there. Could be the connector could just be old wires.
Good job, so far so good
At least the wires on those old trucks will outlast us all
Connectors another story
At the dealer we had connector kits where we could replace the connectors and the inserts
You can just use some good heat shrink insulated butt connectors
That connector is for taking the cab off the frame (or pulling the harness out of the truck) and you really don't need the connector, you can wire around it temporarily until you need to take the cab off
Fun stuff, you're on to it
Thanks again everyone.I'm going back out to look at the truck again now. And to try and get that front left running light and turn signal going. Might be the same connector coming from the front lights causing the problem, but it looks tight. I shall see. Does anyone know about the reverse lights and license plate light? Are those on a different circuit or a different connector as apposed to the rear lights and turn signals? A fuse maybe? Thanks.
Last edited by Bubba5320; Feb 14, 2026 at 05:51 PM.
OK! Finally got the running light and blinker on the left side to work. Bad socket. The only thing left is the reverse lights and the interior courtesy light. Any suggestions much appreciated.
My interior light switch at the driver door likes to be twisted a few degrees or pull pressure applied, then it comes on, I suspect a good cleaning is in order. The passenger one has been DOA since the1980s anyway. I usually just twist the **** on the headlight switch, but one day I might attack the button switch.
Reverse lights are powered by fuse #11 in my fuse box, along with W-washer and turn signals so if they work, fuse is OK. The reverse lights on my '77 with automatic are switched on when the neutral safety switch sees reverse selected, it's mounted top of steering column near the floor inside the cab, two black wires with red tracer are for reverse, power in on one, power out on the other when positioned right.
Great info! Thanks very much. I think I’ll tackle the interior light first. And then look for the switch and wires and see if the switch is working thanks again for all the info
X2 on the dome light check the fuse and of course the bulb. Then the head light switch where the connector sweep arm makes contact with the rheostate spring ring. You can try to clean it, but almost easier to just replace the headlight switch. Black arrows are the contact arm and the spring.
Yellow button to push in to remove the headlight **** and shaft (all in one) and you will turn on the headlights on in the process, so disconnect the battery. Dash/instrument bezel has to come off to get to the nut that holds the HL swith into the dash.
FYI on interior lights with doors closed and how to turn up and down the dash/instrument lights brightness.
Where the NSS/switch to turn on the reverse lights, should be located. Switch might just need adjusted. Careful the 2 short screws strip out EASY.
When the arm comes off and you get mad and just cut the wires. Then truck will not start, unless you bypass and connect 2 wires.
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