Two problems related to horn assembly have been reported for the 2007 Ford Mustang. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2007 Ford Mustang based on all problems reported for the 2007 Mustang.
The Smart junction box on my 2007 Ford Mustang short circuited due to a manufacturer's defect which causes the passenger side front kick panel to flood. There is a rubber stopper which regulates water flow into the engine compartment during rain storms. The stopper is located under the windshield wipers near the cabin air filter. When the stopper becomes clogged due to the elements, it floods the area and sends water in through the cabin air filter. The water will then pour directly on an electrical unit - the Smart junction box. The Smart junction box controls all the lights in the car, the horn, the alarm, and a few auxiliary items. Due to the manufacturer's oversight my car has a malfunctioning horn and alarm. I wrote directly to Ford and called their customer care line about this concern; Ford does not recognize this as an at fault safety issue at this time. Multiple forums that I've found online will support that I'm not the only one who has experienced this safety issue. Thank you.
Horn stopped working approximately 3 months before diagnosis. The dealership diagnosed a water intrusion that damaged the main wiring loom and Smart junction box. It was determined the water intrusion was a result of loose grommets and/or sealer skips around the cowl area, loose a-pillar window weather stripping retainer, misaligned body harness grommet through the firewall or misaligned body harness grommet from the passenger door as per TSB 08 26 7 and TSB 07-14-06. Total cost for the repair was $3,839. 44—just to fix a horn.