2000 Buick Lesabre filter

See my reply to the question: "No warm air from the heater" for details. My answer here is, if you don't use a cabin air filter, a lot of road grit, dirt, fine leaves, dust and diesel fumes will coat the heater fins, and the heater will not give out heat, or no heat. An animal might even build a nest in there.

If you now have a car without a cabin air filter, I suggest you look down into the heater, after removing the cover of the filter, and use a flashlight to inspect the fins, to see if they're covered by grime or dust. If you cannot see down inside, I suggest carefully reaching down, to feel the fins. But try to determine, before reaching inside, if there's an animal in there.