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I HATE splicing/cutting/messing with a factory harness. I ordered this for my '77. Unplug factory connector, then plug this in the middle. I made sure all pins were clean and corrosion free. When plugged back together I even used a bit of silicone or liquid electrical tape to run a slight bead around the mating surfaces to seal it even better. For the ground wire I made it as short as possible to the frame, drilled the hole, then ground down around it for a better connection. After the ring lug was bolted in, I then sealed the crap out of it with sealant to make sure corrosion doesn't kill the connection.
(I also don't like any of the butt splice connectors or things like that. Can't think of how many times I've had to fix someone's trailer wiring because of some sloppy connector had corroded and then corroded the wiring. If I do have to splice a wire, I solder it together then use heat shrink and sealant on it. Never had an issue.)
So then I had a 4 way connector. It took forever but then I found other needed adapters for what I needed.
Basically I have the 4 way behind the rear bumper, that plugs into a harness I made that goes into a 6 way in the rear bumper and a 7 way up in the bed for a 5th wheel.
It is very hard to find a 4 way to 7 way adapter because you would still need to run 3 wires manually. I ran a charge wire and the brake wire all the way back in wire loom. The charge wire needs to have fuses or self resettable circuit breakers.
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