You can do this faster and cheaper on a w124. I bought a 2-channel momentary output remote pair and installed the receiver in my trunk. Tapped into the three wires going to the trunk latch, to get + and - for the receiver power, and one line controls lock and unlock. Grounding the wire (via output relay) locks, and providing a +12 to the wire (via the other output relay) unlocks. (I may have confused which is which.. but you get the idea).

Key still works everywhere as it should.

Don't care about the alarm since I've disconnected that, but I imagine this mod would work just fine. It emulates un/locking the whole vehicle from the trunk using a key. So if this key action would normally un/arm the factory alarm, then this model probably does the same using remote.

I haven't used a key in the door tumbler since...

The remote even has a lock / unlock symbols and is keychain style, and get this.. has metal and a slider cover... it's meant for vehicle application you can tell.

ebay... $10 GO CHINA!

working strong 2 years later

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Cheers!
Scott McPhee

1987 300D