I am trying to figure out how to install a manual HVAC control system to replace the ridiculous "automatic" system in my 1985 300D. The service manual on CDROM is of no help. I am trying to find out how the AC system modulates the amount of cooling delivered to the passenger compartment. Other cars (those with HVAC systems that actually work) have a bypass arrangement that puts all of the air through the evaporator when the blue/red dial is set for maximum cooling and then bypasses some of the air around the evaporator when the dial is set for less than maximum cooling. Does my 300D work that way? If so, is there a name for the air flap or damper that makes this happen?