The usual culprit is a leaking heater core.
The windows fog up with this problem, too.
The fix: Replace the heater core.
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Many heater cores are difficult to reach if you don’t know what you’re doing.
Heater boxes are typically designed with drainage pipes that allow condensation to drip out.
The fix: Drain the heater box and remediate any lingering odor.
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The first step is to unplug the heater box drain if it’s clogged.
Seek professional help if it’s too difficult to reach.
If water gets into your car through a leak, find the leak and stop it.
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Then, let everything dry out naturally or with a heater.
The fix: Locate the component that gets hot or is failing, and replace it.
If the smell comes from the heater, diagnosing and fixing the problem requires access to the heater box.
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Inspect components such as the blower motor to determine which one caused the smell.
Late-model vehicles that use cabin air filters prevent this, but it is possible with many older vehicles.
To prevent this situation in the future, install fine wire mesh over the fresh air intake.
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A fire in the heater box or behind the dash resulting from a bad blower resistor is extremely dangerous.
If you don’t have the means to put out the fire, contact emergency services immediately.
The fix: Leave the fresh air intake off until you determine and address the root cause.
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You might find nesting materials in the heater box or blower motor squirrel cage as evidence.
The critter has done its business in the fresh air intake, heater box, ducts, or elsewhere.
Consider installing some kind of mesh to prevent this from happening again.
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