There's no such circuit, it's all mechanical. The door and trunk lock switches send a 12V or ground signal to the pneumatic pump depending on switch position. The pneumatic pump has internal mechanisms to to await a lock signal when it's unlocked and an unlock signal when it's locked.

I seem to remember the name of B. Greer associated with finding the simplest solution to RKE in these cars. Expand your search to find solutions for 300E or W124. Same central locking system.

Oh, and if your factory alarm is still active, you'll really need those instructions since there's a separate switch in the door and trunk locks that activate the alarm.

Sixto
95 S420
87 300SDL
83 300SD