'98 C230 - air blows out of side dash vents only, except when defrost is engaged, at which point it comes from side and windshield vents. Absolutely no air flow from center vents. Dropped right lower dash panel and removed glovebox to inspect recirc air flap and center air flap actuators - absolutely no motion noted, regardless of PBCU setting. Exchanged vacuum distribution block and PBCU with units of known quality but problem persists.
Appears to be no vacuum within system. What is the source of vacuum for this vehicle's HVAC pneumatics? Any suggestions?