1995 Audi S6 Question

Heater problem
1995 Audi S6 Question All Wheel Drive Manual 60,000 miles

I have dead mice in the heater box (or close to it) of my 1995.5 Audi S6. The smell doesnt happen when the AC is on so that should narrow the problem area to the heater box or the plenum or ducts that conduct heat into the cabin. My question is:

Is there a way to access the heater box from the top, through the engine compartment? I realize that to change the heater core one has to go through the dash to access all the fittings. If I can access the box from the top, would I be able to drop a modified vaccum hose into the box from above to suck out any nasties? Or will the close tolerances in the box prevent such action? Any help on this subject would be appreciated. Thank you.