Welcome to the forum. I have a 190E, 1986 also that I just purchased last month. The sellers had had the alarm disconnected like you want to do because of (they said) a battery drain. It may have been erratic also like yours, and I do not mind not having an alarm. The thing to unplug is
behind a plastic panel toward the front of the passenger side in the front seat - the area where the passengers feet would be.
The way to get at it is as follows:
1. Slide the passenger seat all of the way back.
2. Remove the carpeted mat. It may be glued a little bit somewhere, mine is torn a little on the underside padding but does not show at all from the carpetted side.
3. You will now be looking at some plasitic panels, one horizontal and one angled that is toward the front of the car.
4. The horizontal one has tabs that go into slots.
It may have to come out first.
5. Remove one screw to remove the angled plastic panel.
6. You now can see a couple of metal boxes. The lower one is the cruise control amplifier. (mine does not work; that's how I know all of this.) The upper metal box is the alarm unit. On mine both cables going to the upper box are unplugged.