On MB's you can, as you've found out, tie into the electrical portion of the vacuum operated system, triggering the system using relays.
The other method is to use an actuator in the door to operate the pin up and down, replicating the operation of turning the key.
On later cars with a dash mounted switch, it's simple to tie into that switch.
If your installing shop is unfamiliar with how MB door locks work, then run away. Working on German cars takes a certain expertise. Your door panel is an expensive piece of interior body-work, not some el-cheapo Chry-co part.
And don't go probing wires under the dash without knowing where your SRS wiring lies.
1998 C230 "Black Betty"