that is the security system switch. I dont think that the early cars had a convinience module like the later w124 had.

In those cars the security is spliced to the convenience module and allows you to use the radio and also windows if the door is just unlocked and opened. Allowing use without the ignition.

Might be a time to do some digging wiring diagrams to see what it is in that model.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)