I am posting this message. I have a 92 300E some time ago I had an incident were I discovered a none factory installed alarm shorted out the vacuum door locks . after I had this united disconnected and the wiring put back to together I disconnected the negative battery cable for a hour and it reset the vacuum door lock.
I had not discovered that the trunk vacuum for all the doors, gas cap would not lock via vacuum when the key was inserted and turned. It would only lock the truck manually.
What should I look for , Fuse vacuum unite in trunk were is it located. Other possible causes.

I have a good fluke 88 DMMM and a power probe -3 . I also have some of the wiring diagrams. I just need a suggestion on were to start my investigation?

Bob Geco