If you are sure there are no exhaust leaks then I thnk it has to do with some strange aerodynamics. The extended, down facing exhaust tip sometimes helps. But I have found - in our '84 Spider - that if I drive with one window partially open I smell exhaust fumes. If I partially open both windows it goes way. Plus, even when I don't need heat, I'll often run the blower to supply fresh air from the cowl vent into the cabin. Even with the fan off, forward motion should push fresh air in. So check that fresh air can come in that way.