Have the same car- had the same problem.

Remove passenger footwell carpeting and there are two multi-prong plugs going into the control box, which is on the left side of the footwell. Think it
says becker on it.

Remove both.

The red plug which looks similar is the airbag connector, which is in the middle of the footwell, so don't disconnect that one (obviously).

You can test to make sure the alarm is disarmed by rolling down a window, locking the doors with the key and then pulling the plunger up- which will sound the alarm when it is working properly.

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E