Originally Posted by chris123
The trailer lights work when connected to another harness.
Another harness meaning to a different tow vehicle? Also, the lights you are speaking of, are they the interior cargo lights or the clearance lights?
Two things come to mind. The trailer plug is wired wrong or not per standards. Check the specs on which wire connects to each terminal inside the plug, or the ford is not wired correctly.
Second, if it is the interior lights of the cargo trailer, the ford needs the 40 amp fuse installed in the panel. More than likely the current fuse is not blown, rather it is usually an aftermark installation. They do that to prevent accidental shorting for those who do not plan to pull a trailer.