In-car fumes

[victor norman]

Hi - subject is a 2000 A4 Quattro. Problem is fumes in the car, not particularly strong but enough to make the wife complain. Hard to pin down exactly what they smell of, certainly not like (burning) coolant but since it's more obvious when the heater/fan is on I'm thinking it's a very slow leak in the heater core. Not getting any windshield fogging nor any obvious drop in level in the coolant overflow reservoir.
I guess I have two questions - is it possible to look at the heater core without tearing the dash apart and can anyone recommend a MILD stopleak product?
Make that three questions - if it's not the heater core any suggestions on what else it could be?

Thanks

Vic Norman 2000 A4