I don't have a 124 so my experience might not directly relate, but I just bought a cheap keyless entry system off ebay and wired it up to one actuator on the driver's door (which activates the power vacuum locks). It wasn't difficult.

There's more info on this board (probably some 124 specific stuff) if you search carefully. Hope that helps :-).

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)