I have a 92 W350 that the po wired a brake controller and 7 blade socket up to. When I put the fuse in the link for the aux wire the trailer brakes lock up. Remove the fuse the brakes release but cannot get them to work through the controller or foot pedal. Help!

 

I'll assume it's wired correctly.... Some brake controllers have a leveling control this may be out of adjustment, that alone wouldn't do it but the brake light switches have a bad reputation and it may be one set of contacts is bad, do you have cruise control and if so does it work? If not try changing the brake light sw they are cheap. Second thoughts I reread your post and I don't think it's wired correctly, I would re do it.

 

Naw, the PO wired to his miswired trailer.

Grab a meter or test light, a helper, a kosher diagram, and sort it out.

 

Does the brake controller light up when you put the power to the auxiliary? Had a broken ground wire do that on a trailer at work.