Probably not helpful but here it goes. I had a buddy back in the day with a early 80s firebird, same situation, he had taken to pulling the fuse until it rained then popping it back in. Not really a solution. I took a look at it for him, for the life of me, could not trace back the problem. So always looking for a cheap inventive solution, as you do when your young and broke, I cut out the need to pull the fuse. I had a this big shiny toggle switch laying around so I mounted it in his dash and used it to kill the juice coming from the fuse. True, it didn't really address the problem, and I had to put a hole in the dash to do it. Ultimately it did the trick and 20 years later we still joke about the "James Bond" switch in his Firebird. So like I said probably doesn't help but for what it's worth its an option.