You can buy a 2-channel momentary remote receiver plus two transmitters with lock / unlock symbols on them on ebay, for about $10. China suppliers.

I picked a unit that came with nice metal encased remotes, with slide covers... higher quality so maybe I spent $15 in total. Blue led indicator.

For w124, reveal the connector at the trunk latch, the wire runs from passenger side tail light to the latch. It has 3 wires, you take off your +12 there and ground for your receiver at the taillight. Wire each receiver relay wiper to (I forget if its hot or ground I have to check) and the relay's normally open contact runs to one of the other wires going to the latch. If you press unlock and get a lock, reverse your connections.

Done.

If you have a working alarm, this also arms / disarms it. It's the same as using your key in the trunk to lock / unlock all doors. Did you know you can do that?

Of course, this depends on having your vac motor and lines all working for the door locks.

__________________
Cheers!
Scott McPhee

1987 300D