Car alarm going off at random
 

Try this: After you have unlocked the car, keep the key in the keyhole, open the car door, unlock the door again, and then get in, and see if it still does it.

try unlocking the car from the trunk.

sometimes i have to do this...it's either a worn key... worn lock or a combination of both..

if i unlock my car from the drivers side and the horn starts... i have to lock the car and unlock it from my trunk

this has happened on two 124's that i have owned.

I think your original question is on track - you just need to get rid of this thing. Your car is not a prime candidate to get stolen. These systems age and just become waaaaaay more trouble than they are worth. Better to just get rid of it. You need to find the alarm control module, probably under the front passenger carpet, and remove it.

Same problem with my 420SEL, Disconnected the alarm
 

When I had a problem with my 420 SEL alarm going off in a heavy wind when parked --- I went under the passenger side floor and disconnected the two heavy cables that constitute the alarm connection system and taped over the male sides to prevent shorting etc. No more problems!! The faulty areas of the overall alarm system can be found, but the time and cost are normally not worth it. Ben Carter

Alarm problems
 

Have also an 86 190D and the factory alarm is just trouble. In front of the fuse panel you have another rectangular box. Lift off the cover and you will 2 red fuses there. Remove those and the alarm is inoperative.