I have a strong exhaust/gas smell in the passenger compartment of my 1987 SEL. I have had it checked out by three different shops that all say the exhaust system is working correctly, with no leaks, so I have no idea what to check next. Is there something in the heating system or someplace else that I should check next? Can barely drive the car without almost getting high on the smell. Please.. any suggestions would be greatly appreciated!!! Thanks