I had the same problem on my 89 Bird...It happened all of a sudden. It would be fine for like months then it would just blow. I checked everything also and nothing worked. I sold the car..but the guy that bought it couldent find anything either. He had to Direct Link it to a switch in the car. Sucks cuz you have to remember to turn it on but when there no choice...gotta do what you gotta do

Also you could hook it to a keyed power so it turns on when the keys turned.