Sounds to me like maybe the temperature sending unit fried, so the previous owner just wired the horn button into the fan relay. Not the right way to fix it, but definatly cheap.
If you don't have it already, I'd recommend going to Ron Ayer's and ordering a
GSF400 Service Manual.
Sure, the scans will probally work, but it's a lot quicker to look something up than to ask on a forum if anyone knows what the thingie with the green and white wire, the orange wire, and the yellow and white wire is! Plus it has instructions for a full teardown and overhaul.