A friend found the heater ducts of one of his cars stuffed with Macadamia
nuts from a Neighbors tree. He figures the mice did it.
My wife had a Nissan Altima that the snails climbed onto the exhaust
manifold. She complained of a smell one day and that was the source.
Her Honda CRV developed a strange noise when the heater was on high. I took
it apart to find a mouse nest.
I opened the hood of my pickup one time to find a snake curled up inside.
There is no way I know of to keep critters out. There are many large
openings for proper air flow and it is a nice warm place after a while. I
would think a barn cat or two would help and D-Con in the engine compartment
would keep the damage down.