I am trying to wire in a 7 wire trailer plug--shud be pretty easy. Under the rear bumper, I have three wiring harness. 1 is feeds the rear truck lighting. This harness is also used to wire the camper (tapped into at the front truck bed). The other 2 harnesses appear to be dead (8 wires)--even though one of these was previouslty wired to a 6 prong trailer plug. I am have not been able to find the fuses/relays controlling these 2 harnesses. Thanks for any suggestions philv

 

An 89 would not have been factory wired for trailer lights or a brake controller so have a look under the hood and dash for anything that doesn't look factory.

 

Conanski--thanks for the response. I have followed the 13 wires from the rear bumper to the firewall--they appear to be all original and still in the plastic wiring harness protector. I did find a "Trailer Relay" on the drivers side - along with other factory relays-- I can tap into the current active plug harness-but I hesitate to over load the wires-as they would then feed the truck, camper and trailer--when if all were used. I thought as the other 8 wires are at the back end of the truck--if not for a trailer wiring plug-what is their purpose? any thoughts? tkx philv

 

The wiring:










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Subford--thanks for the wiring schematics. earlier today-I was able to find a 1992 F250 at a wrecking yard a double plug wiring harness feeding a 7 plug Trailer plug. This harness was plugged into the same style harness as on my Ford. With your provided diagrams and with the harness, I should be able to back trace the wiring in order to locate the trailer relays/fuses. Since all 7 wires are dead--their must be a couple of relays that are shutting off all power. Thanks again Phil

 

sorry to hijack this thread but i was hoping subford could send out another couple pics of this wiring diagram. I have a 87 crew cab that im trying to wire up for electric brakes. i think i found enough info to give it a shot but a diagram would help immensely. the photobucket images are no longer viewable

 

Replaced bad images.