Member Paul Drayton has compiled a user friendly SM.
HERE:
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/Index/617Tindex.html
AND sure enough the vacuum diagram is there (Below)
I put the Legend separately so that it may appear larger and more legible.
#2 in the Diagram is the Ignition Switch Vacuum portion (which shuts off the IP)
[I know, It calls it a cam plate,Which it is also]