first of all use foam cleaner
for the heater core as your car does not have a microfilter. That foam cleaner is sprayed onto the heater core and flow slowly down to the plastic pan and then thru the drains under the car.
Coolant level you can check: park the car every day at the same place and check level of coolant with cold engine, for example in the morning before you start engine. You can remove the radiator expansion tank cap and use a measure for fine measuring. If no loss, no leak.
Mouse: of course there could be a dead mouse somewhere, I have seen pics of dead mice in the cage of the blower fan motor, maybe worth to check that too.Shogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html
Re: first of all use foam cleaner
Thanks Shogun. I will remove the blower assembly so I can really see down in there, check for mouse parts, and heavily foam the back of the evaporator and everything else I can reach. What type of foam can you suggest.
What I can't understand is the it smells very much like exhaust or crankcase odor. There are no exhaust system leaks and no firewall leaks. It is definitely coming theough the heater box.