Eyedoctor20,
Couple of notables in your description.
1. You have a consistant ingredient………….water. Therefore, something is getting wet and causing a short. After launching next time out, check your housings and see if anything is wet in there…………….ANYWHERE in there. Pull the bulbs even! If you find moisture, reseal your housings and then unplug your lights on all launch and loads. Give the water a chance to get out if any leaks develop before closing the circuit again.
The rain issue could expand the possibilities but I tend to think that the higher probability here is a “same source” problem……………….fix one, you’ll fix ’em both.
2. I have owned many Fords and worked around many more and they have a known reputation for screwed up wiring! Really, it’s true! There is an outside chance that something may be in the system itself. But your consistancy of water tends to overshadow that tremendously.
Do you have aftermarket wiring or the “tow packeage” and adapter harness? Because water is always involved, I’d start from the connection to the truck’s system and work backward. Try a new adapter if you can grab one from a buddy somewhere. If it is indeed ALWAYS in correlation to the presence of water, let that be your guide. LMK what you can and how it goes!
The best to ya,
Stillakid2