PeachParts Mercedes-Benz Forum - View Single Post - what is this grounded metal bracket by my stereo (early w124)

jay_bob · #2 06-10-2019, 04:31 PM

Yes that was the alarm trigger wire. It was meant to be plugged loosely into the radio and get grounded by the radio frame. The idea was that if a thug pulled out the radio, the strap fell off, ground got broken, and the vehicle alarm sounded.

The slightly later ones had a 5 pin (latching) plug in the back. If you pulled out that plug the wrong way (with battery but no ignition IIRC) it would activate a self destruct that nuked the radio electronics in addition to setting off the alarm. There was a Mercury battery (the forerunner to the Lithium cells we all know for memory backup) that provided the energy to operate this death circuit. I seem to recall that if you send one of these into Becker for a rebuild they disable this circuit and remove the mercury battery since it is long dead by now.

And that single tail (blue wire/black sheath) with the quick connect is your power antenna trigger wire. This will tie into and work with the blue wire on a modern stereo. If you got a more upscale stereo it may have 2 blue wires, one for amplifier activation (on anytime the unit is on) and another for power antenna (on only when radio is selected).