The following holds true for 108's, but should still be close on newer models.
Where the vacuum lines and electrical enter the lower A post on the driver's side, you should find two "y" junctions. One set of lines yellow, the other black (yellow to unlock, black to lock). One leg of the "y" comes from the master valve, with one leg running across the under side of the dash and the last leg dropping down to the floor.
The leg that drops to the floor should be to the left rear door. You can disconnect those lines and attach your hand pump to check the actuator for the rear left rear door.
If you remove the glove box door and liner, you should be able to cut into both lines and check function of the right side. You can repair the cut with a short section of appropriate sized vacuum hose. You can also pull the trim cover off the passenger side "B" post and access the lines going to the right rear door to individually test that actuator.
If you opt to do the tests this way, be sure to isolate the vacuum reservior but taking it out of the equation. Any leaks will show up very quickly without the reservior (plus you can cycle the entire system in about 3-4 pumps).
Personally I opt to pull the interior door skins, seems like a lot less work that trying to dig out the "y" and cutting and splicing.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.