Okay - yes - there is a secondary fuse box but it's not a fuse - it's a relay. ( Apparently Ford has moved to running most things thru relays instead of fuses! ) That was relay #64 - and we've changed that already, and no where else in the manual does it list a separate fuse only for the left trailer towing turn signal/brake light. They've tested the yellow ( left turn signal/brake ) wire for the trailer - there's no current or power coming from the connection behind the right truck brake/light assembly ( behind the panel ), so the problem it seems it between the relay under the hood, and the wires back to this connection. Those wires cannot be accessed in between because they run in hte paneling, under the seats & trim, etc. Could it be another relay? Should we purchase wiring schematics? Thank you for your help!!