2007 Focus Blowing fuses

My 2007 Focus wagon keep blowing (#30) 10amp fuse. It is connected to the tail lights (when the lights are on), the license plate light, and dashboard lights. I think it is related to moisture as it is worse (keeps blowing the fuse) after a rain. I’ve checked the taillights and there isn’t any moisture there (although when I first found this, both taillight housings were pretty melted). Any suggestions for where to look for the short? Some other discussion groups listed different harnesses, but which one…I have the sinking feeling this is going to be impossible to find (or REALLY expensive).

Has there been any electrical modifications to your car, like trailer lights etc? Did you buy the car new or used? How was the repair to the tail lights done, (you did it, dealer did it, etc?)