Find an easily accessible wiring junction & remove one wire at a time. With both the horn & starter being affected, it may be a heat-related fusion of wires. Look around the starter area, work your way back. See if some wires may be melted together? A short means you'd be blowing fuses, it sounds like you've got other types of issue(s).
Sorry I wasn't more help...............
From: "BMWMCON - Tech" <firstname.lastname@example.org>To: email@example.comSent: Monday, June 20, 2016 10:07:46 AMSubject: K100LT short. <<$150988446050$>>
I have gone as far as I can in finding what i suspect is a short on the bike. It is a 1988 K100LT. Had the warning light on when the key was off and the shutoff on. After placing the battery in recently, the horn blows continuously and the starter will not run.
Looking for someone or a shop that can find the short. Trying to get it back up and running in order to sell it.