I'm wondering if anyone has seen this problem. Tow vehicle is a Ford Expedition with the towing package. Problem is with the trailer lights. Left brake/turn signal does not light. Regular tail light does, in fact all light on the trailer are correct, except for this issue. Bulb is okay. Connect is the standard Ford 7 pin blade unit. Connecting to an adapter for the four pin flat connection on the trailer. The wire is going to the brake light is not hot when the brake is depressed, other one works fine. Continuity is good on the wires. Something is wrong with the Ford connector or I'm using the wrong adapter, but it appears to be the correct one.
Anyone have this problem, before I start taking the Ford apart??
Appreciate you help! By the way the trailer has 5 wires into a four unit flat plug. Two wires in one hole.
Bob

You need to check the wire at the 7 pin & 4 pin sides of the adaptor. It's possible the adaptor is bad.

check the fuse box diagram in your owners manual - can't be sure about yours in particular but Fords usually have a separate circuit for each trailer brake/turn