Towing Issues: 7 pin wiring problem, marker lamps, trailer plug

Question
Vehicle '01 F250 w/ towing package

Trailer: 7 pin plug
Marker lamps are one-wire & ground through mounts
Tail lights are three wire (stop, turn, tail)

Problem: when I turn on headlamps, the rear lights act like I am braking and are fully luminated. The disconnection of the running lamp circuit prevents this.

I think it has something to do with the running lamp circuit but after isolating all the marer lamps, no luck....

Any ideas? Thank you.

Answer
well sounds like something is crossed in the wiring somewhere... Is the truck plug stock or an add on ?? that could be the key part.. Most vehicles and trailers are wired the same way but it seems that somewhere its been rewired or you have a wire that is crushed and making contact with another one in the wiring ..  Electrical problems are always real hard to track down , it just takes time, LOTS OF IT...
There are several places where you can get the wiring diagrams for trailer plug connections online, I would do that and then check and see if each one is correct..