Three problems related to horn assembly have been reported for the 2003 Ford F-150. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2003 Ford F-150 based on all problems reported for the 2003 F-150.
Was heavy rain outside then my truck started blowing the horn, heater was turning on and off, lights going on and off by them selves. Figured its part of the issue with the windshield issue Ford knows about since 2002. The notice was *Ford special message #15773 which covered a few vehicles but has expanded to cover other vehicles. Last year, I was on the highway during heavy rain and all the sudden, all power went off, windshield wipers stopped working, radio was going funny, heat was turning on and off, it was difficult to drive. That only lasted for a few seconds but made it where I got off the highway, made it back to my house then brought it to Ford to find out the issue. I opted not to have the work done at that time for the mechanic said its okay for the time being. But corrosion was starting and I needed to have the windshield fixed before the issue at hand could be fixed, because if I didn't, it just would cause the issues a few years down the road again. Looking through, I see you have a lot of "electrical issues" being brought up, and all of them from what I have seen look to be caused by the same issue, mind you that this issue is known from vehicles 1998 - 2003, possibly into the 2004.
The contact owns a 2003 Ford f150. While the vehicle was parked the contact stated the horn began to blow on its own and black smoke began to escape from the windows. The fire also began near the fuse box. The contact extinguished the fire with a fire extinguisher. A fire report was filed. The insurance company had not concluded there investigation regarding the cause of the fire. The vehicle was not repaired. The approximate failure mileage was 62,000.
Consumer complained about an electrical system problem. Inside lights would not go out once the door was closed. Alarm system did not work, and side reflector lights did not come on. The door ajar light stayed on. Door ajar light stayed on for about 20 to 25 minutes before they go off. Doors won't lock when they're supposed to. Dealer said that they can't locate the problem with inside lights. However, dealer made repair on horn and replaced fuse for alarm horn.