Rear running lights on a 1978 e150 are out (lots of info of what I allready tried)

My rear running lights have gone kaput.
The following is what I know Front running lights work. The headlight switch has power comming out of all wires when everything is
supposed to be turned on, so the switch is fine. Both driver and passenger lights are out, as is the licence plate and the small lights on the sides of the van. If my previous information is right the front running lights, rear running lights, and the horn are on the same circit/breaker. The horn still works. I did some testing with a test light. One with a alligator clip and a point to push into the wires Right next to the steering column are two connecting plugs one is three prong the other is 4 prong and they both come right out of the firewall and lead to the rear. The 3 prong I think is Brake+signal/Brake+signal/fuel gauge The 4 prong has 2 light purple wire's, a black, and a brown the black allways seems to have power the brown only when the running lights are turned on(supposed to be turned on) The other two never had any power At the back all the running lights have a brown wire going into them but no power.
The brown wire goes into a weaved insulation with a bunch of other wires and goes down past the sterring column and under the van and to the back. Once at the back I cannot get any power from any of the brown wires I followed the line containing the wires all the way to the back and did not see any other connectors that may have uncliped
Now does anyone know if I have the right brown wire ? And or any suggestions? What are the purple wires for in the 4 prong plug ?
- Eric

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