I've never had to track down a vacuam leak on my 124 car. I would start looking at all the rubber fittings. These seem to harden and crack with age.
Also, do some searching on earlier posts. There was one a few months ago with some suggestions by Steve Brotherton. If I remember correctly, he talked about a rubber connector that was either under the dash or passing through the firewall.
I would exhaust underhood possibilities before going under the dash.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in