I checked the W124 CD manual and couldn't find the anti-theft system in the wiring diagram for the 86-92 series. I checked the W126 CD manual and saw that a YL/BU wire sends a momentary (as long as the key is turned) ground to pin 6 of connector 2. A GN/YL wire sends a momentary ground to pin 7 of connector 2.
If you want to hear from someone who really knows for sure just say so and I'll pipe down
I take it project 500TE hasn't begun?