I would start looking for an exhaust leak. The air intake for HVAC is at the base of the windshield. If you are getting fumes in the car, you should be able to find the leak with the hood open. Listen and look carefully.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.