i would get this checked immediately. i had the same problem and there was a major leak in the fuel lines and the car could explode at any time. i was sitting on top of a ticking time bomb. Dealer had to fix it. http://www.autozone.com/servlet/UiBroker?UseCase=S001&UserAction=viewSimpleDiagInfo&Parameters=info
i believe the heater core is leaking, a small pin hole leak can generate a lot of smell.
Most of the time it is caused by a hole in the heater core, do you smell a sweet smell? The sweet smell would be the antifreeze.
Probably - your head gasket is blown. Antifreeze will be coming out of the tail (exhaust) pipe.
If you smell antifreeze in the passenger compartment of your car your heater core is probably leaking. It's under the dash somewhere ( depends on the make and model ). Another symptom of a leaking heater core is moisture build up on the inside of the windshield when you run your defroster.
heater core possibly plugged and leaking
the best possible guess I could think of is mice... If it's a chemical smell, you might have a heater core leak. If it's a mildew smell, maybe you have an interior air filter that is funky.
Defective heater resistor pack.
Leaking heater core
BG chemicals and some other companies have a cleaner that is sprayed directly into the evaporator case. It would have to be done by a repair shop.
If the car heat has a fresh air setting, then it could be drawing air from outside of the vehicle. The fumes are most likely coming from a problem under the hood.
Probably a leaky heater core. Look in the passenger side floor board (maybe driver side as well) and see if the carpet is wet. These are all tell tell signs of a heater core that is leaking.
Check your heater hoses, that's where the water ciruclates into the heater from the water pump.