I just installed a keyless entry system in my 1985 300CD.
I only used the master actuator to operate the vacuum central locks. Now both of my w123's are keyless.
I cut the wires that are used for the slave actuators out of the harness to lighten the wire bundle.
Here's the door and the red lines show the position of the master actuator. You can see the holes for the screws too. I only had to drill the top hole. I used a few washers on the top screw between the door and the actuator to hold it at the proper angle so it wouldn't bind with the door.
Here you can see how I connected the actuator push rod to the door lock linkage. I had to cut a small piece of the push rod and tape it to the door lock linkage so the connector bug would have enough to grab onto. (The door lock linkage is a thin strip of metal.)
I tapped into #2 on the fuse block which is for the clock and I grounded to the E-Brake pedal bolts. The receiver is zip tied under the knee panel below the dash. It was a PITA running the wires through the accordian at the door hinge.
I got the Keyless entry kit on ebay for $35 shipped and it comes with 2 fobs.
http://cgi.ebay.com/ebaymotors/REMOTE-POWER-DOOR-LOCK-4-DOOR-KEYLESS-ENTRY-SYSTEM-M_W0QQitemZ120353608581QQihZ002QQcategoryZ33723QQssPageNameZWDVWQQrdZ1QQcmdZViewItem#ht_4715wt_1083
The CD was easier than the TD to hook up the linkage. I had to drill both holes in the TD and the push rod needed creative bending to make it work. I'll take pictures of the TD's linkage the next time I have the door panel off.