I'm seriously terrible with wiring, trailer brake trouble shooting.

This shouldn't be as difficult as it is, but i'm slow so here we are.

Truck is an 01 F250. Brake controller is not recognizing that there is a trailer hooked up. Brake controller is verified good,so must be wiring. I found that the blue wire (trailer brake) is corroded and open on the OEM truck wiring. Cut, strip that back, test it with a power probe, (NOT providing either ground or 12v) and the brake controller pops up with a gain number. However when I go to test the wiring, Plug everything in, jump the blue wire to the trailer plug pig tail it doesn't work.  

What am I missing here? Everything else works, it's just the brakes. 

Do you have them mirror image? Common to look at the diagram and get it backward. If not it is a ground. Its always a ground. Some rely on the connection through the ball which is not great if the trailer does not get used much and it or the hitch is rusty. So be sure you have a good ground from truck to trailer.

Agreed on potential ground issue. 

I'd attempt to jumper in a good ground just to verify first