Gas fumes

I have had my 19997 Voyager serviced 4 times in the last 5 months for gas fumes coming into the cabin when the heating is turned on. Each time a repair has been made the problem stops for a week and then starts again. Other than the obvious, my mechanic is ripping me off, I'm not sure if this is an ongoing fault in the vehicle or if the repairs are simply being done incorrectly. Not being very car savvy I don't really know what causes this problem. Any suggestions would be helpful.
Thanks.

I have had my 19997 Voyager serviced 4 times in the last 5 months for gas fumes coming into the cabin when the heating is turned on. Each time a repair has been made the problem stops for a week and then starts again. Other than the obvious, my mechanic is ripping me off, I'm not sure if this is an ongoing fault in the vehicle or if the repairs are simply being done incorrectly. Not being very car savvy I don't really know what causes this problem. Any suggestions would be helpful.
Thanks.

1. Exhaust manifold gasket leaking.
2. Check the seals between the engine compartment and the heater/air conditioning box.
3. Check for proper sealing between the air inlet duct assembly and the dash.