Anti-Theft gone haywire
I've had the same problem with my 88 560sl. I've had the car for almost 4 years now, and the "haywire" problem started happening to me 1 month ago. I had to disable my alarm, and it was a 2-step process. There is probably a better way, but when my alarm started going off all by itself at 6 am one morning when I unlocked the door and opened the trunk, I couldn't turn it off. Closing the door, locking and unlocking didn't help this time. I had to cut one of the wires to the horn to get it to stop. There are two wires that go to the top terminal of the horn. One will be thicker than the other one. Cut the THICK one. This will allow your horn to still work, but the alarm will no longer sound. You will need to remove the 2 fuses from the 1"x2" black box that sits right in front of the horn. The cover just pulls right off.
I hated to disable my alarm, but I couldn't keep waking up my neighbors. It would go off while I was driving also.....very embarrasing!
If anyone knows a better way, please let me know.