Ford: wiring to pull a boat trailer, fuse box, turn signals

Question
I previously used my 1998 Ford F150 to pull my boat. At that time the wiring for the lights and turn signals worked just fine. Now, they do not work. I've installed a new harness and plug and checked the fuses in both locations on the truck. The lights work on the trailer with another vehicle, so I know it's the truck. What else could be wrong and is there anything else I should be doing?

Answer
did the problem happen after you hook up the trailer or is it happening with the trailer hook up?also is it doing it with the trailer disconected.make sure that when you ck fuses for the lights that thay are on because the head lamp switch supplys power to the fuse box. let me know i'll walk you a pinpoint test..