Horrible, Awful, Stinky, etc......FURNANCE HEATER SMELL!!!

My bet would be mouse urine. The mice may have fouled the ductwork and/or peed on your furnace heat exchanger. Nature's Miracle Urine Destroyer works well to get rid of the smell. It's an enzyme, not a deodorant, so it has to come in contact with the urine residue, but it works. Cleaning the ducts can be accomplished with an electrician's fish tape and a rag, the furnace will have to come out and be partially disassembled.