I used to have an '88 Bronco eddie bauer, i had the same problem, i had a radar detector hooked into the cigarette lighter and that didn't work either. it was a fuse for me, but i'm not exactly sure which fuse it was, you could check in the owner's manual for the fuse chart, i'm not entirerly sure which fuse it was because it happend a couple years ago.
yeah they do share a fuse but take out the glove box and reach into the right of the dash and there is somthing that looks like a little box with like 5 prongs in it and that could be why the horn is out too but try it it might help maybe
I would sure look at the trailer wiring. At my shop the majority of tail lamp repairs with trailer hitches, is over 50% caused by the trailer wiring. I would definatly check it out first. Then If that all seems OK pull the tail lamp assembles out and look at the sockets and bulbs. Any of them leak water??