1995 Chevy Beretta Smells when 1st cranked

Each time I crank the car it emmits a stroung smell form the Heating unit. It does not do it under defrost or any other setting just the heater. It does not smell light antifreeze but more like a gas/exhusalt smell. We had new "O" rings put on the injectors, new hose and filler for the canister that works the controls. It is my wife's car and she is about to have a cow if we don't find the problem. Can you please give of something else it might be.

Thanks