Factory trailer wiring

[Kevin 007]

Was there such thing as a factory tow package/trailer plug on slantnose rigs? The reason I ask is because the harness from my 7 way trailer plug connects to the truck harness in a very neat/professional manner with plugs that look like they are factory. All the lights work as they are supposed to except the running/tail lights. Im trying to find where the 7 way harness has been spliced into the tail light wiring but can't seem to find an obvious splice.

Is there anything I can look for?

 

[franklin2]

Yes they did have a factory tow harness back then. Yours has a relay for the trailer running lights, but I am not sure of it's location. I would start by looking around the brake booster area and on the driver's side inner fender for any mysterious black boxes that could hold a relay. Power for this circuit comes from a junction block that should be over near the starter relay/passenger side fender. It's a red wire that goes into a fusible link, and then turns into a black/white which feeds the relay. This is relay is the same one you would have if your truck was a dually with the extra rear running lights.