So just this morning, there was a bit of a fog...
and then my left tail light signal quit working. I first noticed that the indicator light on the dash was blinking very fast and my first thought: "my tail light is out". But looking at the bulb, both filaments are intact and once the fog cleared and the sun was bright and high in the sky, the signal was working fine. The indicator in the dash was fine and the bulb actually worked.

But, it started to rain in the late afternoon...
and the signal in the left rear quit working again and the indicator in the dash was blinking fast again.

I do have a hitch cover that lights up "FORD" when you hit the brakes, but I disconnected it and the light still does not work.

Only rarely does the turn signal work in the rain, but only for a second.

So, bad bulb or moisture?

Specs:
06 FX4 Level II
45K miles
All stock

 

Id say that the moisture is causing a short in the circuit. Try water proofing it as much as possible with tape and what have you, and then hose it down nice and good and see if it fixes it.

 

replace the bulb..1.00$ FTW if it doesnt fix it your looking for a moisture problem, somethings "shorting" it out and not letting it work, or if you really want..

using the bulb that wont work, unplug it, grab a test light and leave your signal on, put the test light on either pin and if it lights up on one and flashes its working at the connector and the bulb is toast..NOW if it doesnt work then the problem could be farther down the line..check the wiring..not that hard to find might have to play detective!