The alarm goes off without opening any doors or trunk lid? What happens if you unlock the car at any locations but do NOT open the doors or trunk lid? Then lock it back again, any alarm?

I think all the switches at the vacuum pods are the same. It only has 3 wires, blue, red and yellow ( IIRC ). It is used to detect +12v or 0V. +12 would lock and 0V would unlock. The sensors to detect intrusion are at the door hinge, trunk lid and hood. Therefore no alarm should not go off if you do not open anything.

Anyone messed with the trunk light wiring? The sensor circuit is there.
Anyone messed with the door hinge sensor. I think there are 2 at the hinge. one for the courtesy light and one for the sensor. It is possible the sensor switch is faulty.

This is what I would do. Hardwire the sensors at the door to close position and see what happens. Then remove one at the time and you many find out what is the problem.

If all failed then install a Remote Entry to the car like I do with all my cars.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.