Zombie thread resurrection!
Years later, I finally solved this issue! I actually resorted to unplugging my horn during the summer months because this only happened during warm weather. I recently noticed my parking lights randomly coming on, and one day after my commute home they were stuck on. I unplugged the security module and they went out. Ended up replacing the module and reconnected the horn and all is good now. Turns out it was the security module, just like I had apparently suspected many years ago.
Shoulda done that years ago!
Just wanted to conclude this thread in case anybody has a similar issue and ends up coming across this thread.