Going on 25 years ago my MIL had a Sundance that occasionally smelled when the heat was on. My SIL drove it mostly and she was a bit messy. Several mechanics and shops couldn't figure it out. Some said to replace the catalytic converter, or brakes, other had no real suggestions. I got tired of hearing ll this and decided to get to the bottom of it. I found bird seed in the air cleaner assembly. Then decided to take the heat vents apart. You guessed it, dead mouse dropped out. Problem solved. MIL kept lots of bird seed in her garage. Wasn't the only time seed managed to find it's way into the engine compartment.