Hey Mandi, welcome! I've got a 74 and had the same problem. I traced everything out to make sure it was tight first. Exhaust @ header, heat exchange tubes, heater boxes. Checked for rust through as Marc mentioned before. A big cause is the engine seal not being in good shape, if there at all. Mine is nonexistent and I'm missing my rear tin. That allows mass fumes in. I went ahead and cleaned my engine and compartment with engine degreaser. It had a bad oil plate leak before I got it and had covered the entire bottom of the engine and exhaust with oil that "burned" off every time I drove it. Since cleaning it and fixing the leak I've managed to just about get rid of all fumes. Of course until I get the rear tin replaced I will still get some but my point is I guess, If everything else seems intact, try cleaning up any oil you have leaking. Good luck to you!
'78 Riviera camper
"..at least I'm enjoyin' the ride"