I finally got around to installing the keyless entry system (bought in January, good grief) in my 124. The system is an Omega REC-11 (about $24 on eBay). This system has been discussed in this forum and some good advice posted.
I did a very simple install of the system, just laid the parts on the floor of the car under the rear seat and wired into the pump. The receiver and relays aren't even attached to anything, they just sit on the insulation. I didn't buy the extra-cost relay pack as I had lots of automotive relays.
I didn't use any of the optional extras that the Omega REC-11 supports, like the light flasher or the self-destruct circuits ["Warning: you have broken into a car equipped with the new Omega self-destruct mechanism. Self-destruct will occur in 10 seconds . . .9 . . . 8 . . ."]
As other threads have covered installation thoroughly, I'm not going to repeat. I'll mention only a few notes:
(a) I removed some unused wires from the main connector, including a connection to the "hot in run or start" lead. Without it, you can't program the system, but I found I didn't need it. I didn't use either the LED or the "valet switch."
(b) In my W124, the stock 0.8 second delay time is sufficient for locking and unlocking. Omega gives you an option to change this to 2 seconds (recommended for Mercedes) but so far I've been OK without it.
(c) Owners of "later model" Mercedes with an electric pump for door locks know that you can't lock the doors from the outside with the key in the ignition switch (engine running or not). There's a circuit that is part of the "key-in buzzer" to prevent you from doing this. I think you can go around to each door and lock it with a spare key but that's a hassle.
Well, the keyless remote allows you to lock the car from the outside with the engine running. This allows you to warm up the car in the morning (if you do such things) without getting the car stolen. It allows you to let the car idle (staying warm or cool) while you run into the post office to pick up your mail, again without leaving the car at risk, unlocked.
With my W123, I could do that simply by carrying a spare key. Now I can do it with the W124 also.
Jeremy