1990 E300 D 2.5... A/C and heat work but regardless of the the pushbutton control unit setting, the vast majority of the air is directed to the defroster vents.

The HVAC system derives it's vacuum from the brake booster vacuum line, via a brown plastic hose. Vacuum is present in the booster vacuum line.

The brown vacuum supply line enters the cabin through a firewall grommet on the driver's side. Where does this brown vacuum supply line connect to the HVAC system? On the right side of the heater box? Its not visible with the instrument cluster, PBCU and glove box removed.