'03 Lariat SC. I've got factory wire harness for trailer and I have no left turn signal at plug to hitch plug. Does any one know where the harness plugs in under dashboard? Trying to track down where the break might be. Yes I have checked for voltage with test light, not just plugged in to trailer. TIA

 

Check fuse 20 under the hood.
If you don't have a manual you can download one from the link in my sig.

 

there is also a connection under the driver's side rear tail light.

 

It was fuse 20. Not used to having vehicle with factory harness for trailer. 😎

 

Clad it helped. It's nice that ford uses separate fuses for the trailer harness.