Mercedes: 1986 560 SL, vacuum leak, 420sel

Question
The alarm system on our 1986 560 SL activated and won't reset. A horn and lights flash for several minutes and stop and start again. I have unhooked the battery, negative first and the positive, let is set overnight, re-hooked pos first then neg the next morning.  When hooking up neg post, saw an arc from the cable to the post, so something is still live.  Alarm again went off when I hooked up hte neg cable.  I just got this car a few months ago and don't have an owners manual yet.

My son recently installed a new radio,(not factory), would that have an impact and can we disconnect the alarm without affecting the performance of the vehicle?  any other tips or ideas?
thanks

Answer
Mike
I had a similar incident with my 420sel. I disconnected the battery went around and using the key locked each door or in case of the rear physically locked them.
I then came around and reconnected the battery.
The alarm may chirp but should reset.
Now take the key to the driver door and unlock the door. Make sure you know which way is the unlock director before you do the locking. If you go the worn way, it will cause the alarm to go off.
When you turn the drive door key to unlock wait until you see the remaining door unlock before you try to open the driver door.
Just remember that each time you key lock the car that you always turn the key in the correct direction and wait for the other doors to opem.

If non of this work, I expect that you have a vacuum leak in the locking system. A leak allows the door servos to leak down and the alarm system goes off.

Additionally, Yes I have heard that installing aftermarket components, can effect the alarm system.

I believe that the alarm control module is under the passenger seat or kick panel.

Good Luck let me know how it goes.
Rob