I have a 1980 300TD and have been driving it for 3 years with no working HVAC. The only setting that works is the defogger setting on high and cranks out good heat (i could care less about AC for the summer). I don't want to troubleshoot the system and there's parts missing to the servo unit under the hood. Is there a way I can access where the vents close in the duct work and make the defogger setting blow out the upper and lower vents also? Basically looking where to access that area and just take out whatever closes off the vents from the defogger.

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2022 Nissan Frontier Pro4x
1990 Mercedes 420SEL
1950 Packard 8

Previous German cars-
2001 Mercedes SL500
1983 Mercedes 300SD
2011 BMW 328i (manual)
2008 BMW 535xi (manual)
2006 BMW M3 (manual)
1980 Mercedes 300TD
2006 BMW 750i
1996 Mercedes E300D
1994 BMW 740i
2006 BMW 330xi (manual)
1999 E300DT Smoke silver/black 253k
2012 BMW 535i Xdrive M-Sport
1984 300TD-T 304k
1988 Mercedes 560SEL (parts car)
2010 BMW 550i xdrive
2017 Audi A4 Presitge
1979 300SD 313k
2003 E500 189k