I have a 1980 300TD and the HVAC does not work. I have been using the defogger setting to heat the car for the past couple years since that has its own separate electrical switch. Over the past few months the blower hasn't been operating properly. It speeds up slows down back and forth and is never constant. If I tap on the bottom of the box that contains the fan it will make it run faster. Within a few seconds it will again fluctuate drastically. If I hit the bottom of the fan box, again it will speed up and then fluctuate. Does this sound like a harness issue, or a fan itself issue. I didn't want to buy a replacement fan if there is something else to trouble shoot first?

__________________
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