'94 LX currently ~238k
This problem first appeared about two years ago when it was really frigid out.
For awhile, thought I was breathing the exhaust fumes of those cars in front of me. But after about a week or two eventually caught on to the pattern that the strength of the smell was tied to running the blower on full.
QUESTION: What was the smell ?
ANSWER: Burning plastic.
QUESTION: What was the source of the smell ?
ANSWER: The small and large plastic wire harnesses for the blower motor.
SUPPOSITION: The motor was pulling enough amperage to burn-up the plastic wire harnesses so it must be bad right ?
ANSWER: Nope.
QUESTION: So what caused the plastic on the wire harness to burn up ?
CAUSE: The contacts on the wire harness and those on the resistor assembly were severely corroded. (rusted in fact)
The motor is in fact, good. The bearings are good and turns easily, even when heated. Replacing the motor only provided temporary relief.
RESOLUTION: To resolve the problem, the metal contacts need to be replaced. Cleaning them up will be just as temporary. Ordinarily they should not corrode in that manner. The alloy used in manufacturing them is defective.