Awful smell
 

The last couple of times I have turned the heater/defroster on there is a foul almost dead fish smell that comes in the car. What is it?

mould growing on your evaporator.

This is not an uncommon problem. There are places that can take care of this. What they do is spray a disinfectant into all the nooks, crannies, and passages of the AC ducts to kill the mold. Then another compound is sprayed into the area where the evaporator is to prevent new mold growth. This process seems to work. You can also try keeping the heat on max high for a 15 to 30 minutes to try and dry out the mold. Trouble is, once it gets humid again, it will probably start smelling again.

I guess mentioning your car model/type would help.

Thanks for the information, you guys are all awesome. This forum rules! I am getting the 92 300E hoses mold etc. disinfected at CarX,they actually do that whole chemical process and sounded knowledgeable about the technique.