The heating system has an inside air and outside air setting. The outside air I believe is 20% mixed with 80% inside air. The inside air switch should toggle to 100% inside air. If this does not get rid of the smell, you could have a bad diaphragm that blocks the outside air. This diaphragm is located behind the glove box.

This might be slightly off subject, however if this is working properly it will help with your process of elimination.

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David
1988 560SEL, Green, Chrome wheels.
2000 Suburban, 20" wheels, DVD, wood grain dash (wife's car).
67 Beetle euro, Lots of Empi parts.
69 Beetle conv, 1980s cal look.