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Ok I’ve made some decent progress today. Dave those two photos you shared earlier are starting to click with this caveman.
I have a similar harness of which that “mystery connector” comes out along with a wire that runs to the rear tail lights.
I tested the other day in RUN and wasn’t getting any voltage in the terminals of the mystery connector. I’m wondering if that was plugged into another transmission this truck might have had at some point.
Additionally, I’m wondering if the feed is dead further upstream. The wires don’t look the greatest.
I checked the fuse and it was good.
is there some other test I can do in front of the connector? If it’s just the connector, do I clip that off and run the new plug into the switch to this? I’ll do some research tonight.
Did you check the door tag to see which transmission was built into your truck? Once you know the build the wiring will be the same then look up the switch for the original trans. The switch will tell you the connector type. Which should be a two wire coming off the frame harness. Now it could have been modified along the way as well. Look for cut and paste scars.
Did you check the door tag to see which transmission was built into your truck? Once you know the build the wiring will be the same then look up the switch for the original trans. The switch will tell you the connector type. Which should be a two wire coming off the frame harness. Now it could have been modified along the way as well. Look for cut and paste scars.
the door tag says T19 but I think it might be a T18. It’s hard to downshift into first. The connector looks just like the one from Kr’s post further up. A red oval 2 pin connector
the door tag says T19 but I think it might be a T18. It’s hard to downshift into first. The connector looks just like the one from Kr’s post further up. A red oval 2 pin connector
I'm pretty sure the T19 and the T18 have the same reverse switch. Look up part numbers for both on rockauto.com. the shape is the same oval, so you're still looking for that oval connector.
Did you check the door tag to see which transmission was built into your truck? Once you know the build the wiring will be the same then look up the switch for the original trans. The switch will tell you the connector type. Which should be a two wire coming off the frame harness. Now it could have been modified along the way as well. Look for cut and paste scars.
Originally Posted by 19bulln0se85
the door tag says T19 but I think it might be a T18. It’s hard to downshift into first. The connector looks just like the one from Kr’s post further up. A red oval 2 pin connector
Be it a T18 or a NP435 (truck came with a T18 and I installed the NP435) and I am guessing from the switch you pictured they all use the same plug at the transmission.
So that tells me the frame harness from the main to the plug I showed and then to the rear lights would be the same too.
You need to find what wire in the frame harness that goes to the rear backup lights. Is it 1 of the wires in the 4 wire plug?
All you need to do it put power to that wire for the lights to work.
That is why there is power going to the switch and then out to a wire going to the lights out back.
It is that simple.
Maybe pull 1 of the rear lights and see what the wire color is to the backup bulbs and see if that color is 1 of the 4 at that plug.
Then all you have to do is get power to 1 side of the switch and out to that wire to the rear.
Dave ----
Besides pulling a reverse lamp out to check the wire color you can also look at the EVTM for your year over on Gary's site. Did Max post the link? He has been Johnny on the spot with those handy links lately.
Besides pulling a reverse lamp out to check the wire color you can also look at the EVTM for your year over on Gary's site. Did Max post the link? He has been Johnny on the spot with those handy links lately.
I ordered the connector on RockAuto. It will be here in next few days!
I took one of the taillights off. I saw a bright green and what appears to be two dark greens or blacks. All were solid
none of the four from the mystery connector look to be candidates.
I’ll attach a picture of the mystery connector wires again for reference.
while I was worming around underneath I also found another that the taillights connect to. It was in the center by the license plate area. I’ll attach a photo of that too. Thanks everyone Wiring for top bulb in taillight Mystery connector wiring Another connector by license plate
Last edited by 19bulln0se85; May 21, 2026 at 04:32 PM.
Just found a big clue for you. That plug looks like it was intended to mate to an AOD transmission. Per RockAuto, here's the corresponding view of the switch:
Now we just have to match the wires on that plug to the 1985 wiring diagrams, and then splice two of those wires to the new pigtail for your oval switch presently installed.
Just found a big clue for you. That plug looks like it was intended to mate to an AOD transmission. Per RockAuto, here's the corresponding view of the switch:
Now we just have to match the wires on that plug to the 1985 wiring diagrams, and then splice two of those wires to the new pigtail for your oval switch presently installed.
wire colors don’t exactly match but getting a lot closer.
Don't be misled by the wire colors on an aftermarket pigtail. Ideally they would match the factory harness but there is no guarantee. You have to go by the function of each contact, not the color of the insulation.
That switch had 4 contacts, so likely it is 2 poles (circuits) in a common housing. One pair of contacts has continuity with the transmission in P or N, enabling the starter control circuit only when safe. The other pair of contacts has continuity in R, completing the circuit for the reverse lights.
Since the truck starts normally, a previous owner likely bypassed the starter safety circuit. Where or how the changes were made? No idea. I wouldn't be surprised if he inadvertently messed up the circuit for the reverse lights, too. Either he never noticed or didn't care. You may have to do some sleuthing to make sense of it all.
Ok you are talking "back up" lights and you are looking at the tail & turn / brake light socket. What is the wires to the other socket that is the back up light socket.
IIRC the ground wires are black for the lights so 1 of them 3 wires would be black. Also 1 of the 2 wires to the back up socket would be black too.
On that 4 wire plug that Karl may have found what it is to I got a few questions.
1- I think I said in a post to take a test light to it an see if any of them have power and note the wire color.
2- if this is for a AOD NSS and the truck was a swap to the transmission it has now would that plug no be jumped to get the motor to crank?
If that plug is for a AOD then 1 of them wires would be hot when the key is to start. (pull the wire off the solenoid when someone turns the key to start when checking fo4r power on this plug.
3- when you pull the other light socket for the back up light to check the wire color, 1 should be black, see if that wire color is in the 4 wire plug.
Dave ----
Ok you are talking "back up" lights and you are looking at the tail & turn / brake light socket. What is the wires to the other socket that is the back up light socket.
IIRC the ground wires are black for the lights so 1 of them 3 wires would be black. Also 1 of the 2 wires to the back up socket would be black too.
On that 4 wire plug that Karl may have found what it is to I got a few questions.
1- I think I said in a post to take a test light to it an see if any of them have power and note the wire color.
2- if this is for a AOD NSS and the truck was a swap to the transmission it has now would that plug no be jumped to get the motor to crank?
If that plug is for a AOD then 1 of them wires would be hot when the key is to start. (pull the wire off the solenoid when someone turns the key to start when checking fo4r power on this plug.
3- when you pull the other light socket for the back up light to check the wire color, 1 should be black, see if that wire color is in the 4 wire plug.
Dave ----
Ah, my bad. Glad I took a picture of that one as well. It has 2 black wires. One with a pinkish line. This does look like 1 of the wires going into that connector.
I checked voltage at the terminals with the truck in RUN, a few days ago and wasn’t getting anything at any of the 4. Should I try to jump the terminal associated with that black/pink wire to each of the others to see if I can get the backup lights on?
Last edited by 19bulln0se85; May 22, 2026 at 11:34 AM.
If you did not have power on 1 of them wires in the 4 wire plug then I dont know how the backup lights would of had power if it had the AOD transmission?
I do see what you mean of the black / pink wire at both the socket and the 4 wire plug.
I would hate to put power to the plug Bl/Pk wire not knowing that it is the same wire.
Get some wire and ohm that wire from the plug to the light socket. Dont ohm it with the bulb in as it will show it as the same wire because it goes to ground thru the bulb.
(you dont hear me say this but if you do feel brave you can try and put 12 volts to that Bl/Pk wire at the plug with bulb in socket and see if that light up)
If you do find that Bl/Pk wire is the same then getting power to the switch on the transmission I think will be easy.
Dave ----
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