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I was thinking they come on without the headlights. Seems I see them come on in the daytime. It does signal your intent, like backing out of angle parking. If that's how it's wired then there should be 12v on one side of the connector. It could go through the ignition switch so try in the run position. Or look up the wiring diagram that Max posted. Good luck!
I put the lights on and the truck in RUN/ON. I wasn’t getting any voltage in the cavities of that harness. Perhaps it’s unrelated to the backup lights. When I get to my desk, I’ll take a look at that wiring diagram. Thanks for all of the help everyone
Here's a purloined image from eBay showing the location of the reverse light switch on a T-18. This is the same arrangement as the T-18 on my '84 F250:
Red switch connector at the lower left corner of this stolen image
Not sure about that round connector you found. Are you positive you have a T-18? Look at the certification label on the driver's door jamb for a transmission code. Post that and we can help determine what you have.
Also for the NP435 transmission.
I cant remember and I am not getting under the truck at this time, if it is a short harness from the frame rail harness, extension that plugs in, or part of the rail harness.
It should be 2 wires, power with key on and the other to the backup lights. It would be where the crossmember bolts to the frame rail as the wires run along the crossmember to the switch.
My lights also did not work when I first got the truck on the road. The plug side did not have power but the wire just before the plug had power.
The wire broke inside the plug so I cut it off. I was able to get a weather pack plug, I have a kit, to fit the switch and push it in and I now have back up lights.
Been that way for 6 years now.
Dave ----
Last edited by FuzzFace2; May 13, 2026 at 07:27 AM.
Thank you Dave. That connector I found by the frame rail is circular; whereas, the switch on the transmission is oval shaped. I haven’t tracked down what the connector I found along the frame belongs to yet. There’s a “key” looking cavity along with 4 smaller cavities within it.
Thank you Dave. That connector I found by the frame rail is circular; whereas, the switch on the transmission is oval shaped. I haven’t tracked down what the connector I found along the frame belongs to yet. There’s a “key” looking cavity along with 4 smaller cavities within it.
When I first seen the picture I wonder if it was for a auto transmission for the NSS but that would I believe have 4 wires. 2 for the backup lights like I posted and 2 for the safety switch for starting, not 3 like I see.
Also if the truck starts with the key and that plug is not jumped out I cant see it being for the NSS.
I would check what the color of the wires are and post them as someone may know what they are for as I cant think of anything they would go to with 3 wires and a round plug in that area.
Wait what year / model / motor and if dual tank or not? Fuel gauge working? I know it is not for a 3 port fuel switch valve as that only has 1 wire but maybe the other switch valves have 3 wires?
There has to be 2 wires hanging out on the frame rail harness by the crossmember area.
Dave ----
When I first seen the picture I wonder if it was for a auto transmission for the NSS but that would I believe have 4 wires. 2 for the backup lights like I posted and 2 for the safety switch for starting, not 3 like I see.
Also if the truck starts with the key and that plug is not jumped out I cant see it being for the NSS.
I would check what the color of the wires are and post them as someone may know what they are for as I cant think of anything they would go to with 3 wires and a round plug in that area.
Wait what year / model / motor and if dual tank or not? Fuel gauge working? I know it is not for a 3 port fuel switch valve as that only has 1 wire but maybe the other switch valves have 3 wires?
There has to be 2 wires hanging out on the frame rail harness by the crossmember area.
Dave ----
It’s an 85 with a 351W. It has a single tank.
When I get home I’ll crawl under and get a better look at the wires. Thank you.
edit: forgot to mention the fuel gauge is working
Last edited by 19bulln0se85; May 13, 2026 at 09:13 AM.
When I get home I’ll crawl under and get a better look at the wires. Thank you.
Well then the wires are not for a switching vlave.
If I can get my fat azz under my truck today I will see if I can get a picture when the wire comes out from the rail harness.
Dave ----
I cant remember and I am not getting under the truck at this time, if it is a short harness from the frame rail harness, extension that plugs in, or part of the rail harness.
It should be 2 wires, power with key on and the other to the backup lights. It would be where the crossmember bolts to the frame rail as the wires run along the crossmember to the switch.
Ooh, Dave, you're a genius. I think that is connector C128. See the wiring diagram for the backup lights here on page 81:
If that mystery round connector is C128, the short harness between there and the transmission is missing. Can't remember, did you check for power, etc. at that mystery round connector?
If that mystery round connector is C128, the short harness between there and the transmission is missing. Can't remember, did you check for power, etc. at that mystery round connector?
Ah, interesting! Yes, I wasn’t getting any power at the connector with the lights on
Ah, interesting! Yes, I wasn’t getting any power at the connector with the lights on
The backup lights only get power with the key on not the lights.
So you need to turn the key on to run or I believe ACC. to see if it has power on the wires.
I still dont know why there would be only 3 wires unless I count not see the other wire?
Dave ----
I had to go get under and look at the connector and it is a male? with female pins, ovel with 2 wires going into it = brown & black.
It then plugs into an extension harness to the switch. It runs to the rear along the rail to the crossmember, along the crossmember then up to the switch.
Sorry this picture is not a little better but it was tight being a 4x2 and not jacked up.
Dave ----
Last edited by FuzzFace2; May 13, 2026 at 10:02 AM.
Can you wait there? We'll have some more things for you to check. Might be a day or two but if you don't mind waiting...
I would not mind other than needing to eat and a bathroom but the wife just had an operation and I am taking care of her ATM
I did give a quick look and did not see any round plug.
I should also point out this is on my 81 F100 300 / T18 (now NP435) and dual tanks. The truck the harness came from was also an 81 F100 300 / SROD and dual tanks and why I added to 2nd tank to my flare side.
Dave ----
The backup lights only get power with the key on not the lights.
So you need to turn the key on to run or I believe ACC. to see if it has power on the wires.
I still dont know why there would be only 3 wires unless I count not see the other wire?
Dave ----
Hope you all had a great weekend!
My picture was blocking the fourth wire. I’ll attach another photo. It looks like 2 red, a reddish-brown, and a black.
I also re-did the test following your instructions and still didn’t get any voltage to any of the terminals in that connector. I did some rolling around under there and found another connector relatively close. Maybe a foot away along the rail. It looks like the one in the picture that you shared. I’ll upload a picture of that too.
The mystery connector was tied up where the rail and crossmember meet.
Unfortunately, I just know the basics. I haven’t found any wiring from the engine bay towards the transmission yet. I also haven’t identified any wiring that would plug into the reverse switch which runs to the rear lights. I haven’t given up yet though!
P.S. I’m glad you opted to not stay under the truck until I responded haha.
The truck is an 85 with a 351w manual transmission and single tank.
I dont know what the 4 wire plug is and you say none have power to them. Key off / key on / lights on / etc.?
I dont remember any other plugs along the frame rail but my truck is a pretty basic 300 six / manual flare side.
The backup light wiring is not a harness to the rear, it is only from the frame rail harness to the transmission switch.
A power wire is in the harness and 1 wire to the plug in my picture.
The other wire in that plug goes back to the backup lights.
If you were to jump them 2 wires together in that plug the backup lights would come on.
There is a 2 wire harness from that plug to the switch on the transmission shifter tower and that is the "jumper" when put into reverse.
Just look for that 2 wire plug that I think will be ovel that the "jumper" harness will plug into.
Dave ----
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