The left trailer turn signal on my '00 F350 suddenly quit working this spring. Neither the 7-wire or the 4-wire plug is throwing a left turn signal to the trailer. The other lights, electric brakes, etc., work fine. The truck taillights work fine. I've traced the wire back through two chassis harness plugs, and can't get a signal. Wondering if anyone else has seen this, and where I should look next. I hate to take it to the dealer to trace at $80/hour. Thanks.