Graplr, you're right on the money... I bought a cheap aftermarket system (Crimestopper CS-865-RKE-II, $30) and a $5 universal actuator off eBay. All you've gotta do is wire up power/ground to a few wires on the aftermarket control box (I put it under the driver's side kick panel - ziptie it to the steering wheel bracket), pull the 2 control wires through to the door (the hardest part of the job) and wire them up to the actuator. It gets a tiny bit more complicated if you want flashing parking lights or a few of the other features, but I'm not going to bother with that.
Here is a photo of the actuator. If you want a photo of the control unit I can take that too, but I don't think seeing it would help you any and it's kind of hard to see where I have it mounted.
Earlier today I pulled the smaller stock lock switch from my 500sel and compared it to the one in the 300SD. It's smaller for sure, but not in any way that would help me squeeze an actuator next to it. One idea I had is that this switch might be able to be replaced completely with a universal
5-wire actuator (basically the same as the 2-wire actuator I was trying to install, with a built-in a 3-wire switch like the stock one in the car). Since I didn't try this, I don't know if the switch in the universal actuator would be compatible with the stock wiring and vacuum pump.
