I had the same problem on my 87 300D. I remember thinking it was the alarm also! Seems theres a wire connection in the passenger side rear floorboard where the wiring harness corrodes because water gets back there inevitably. What I mean is pull up the carpet on the rear passenger floorboard and look for the wiring harness that runs back to the vacuum pump. Theres a metal connector that corrodes.

This drove me nuts for a year until someone told me the fix on the Forum. i went right outside and saw it and I was elated because a 1/2 hour later I had it fixed! It all clicked because once I drove through a rainstorm and suddenly my vacuum locks worked for several days thereafter.

A Mercedes is not a Mercedes until yoy have the vacuum locks running!~~~~~~~Please tell me if this helps!