Were you looking at the heater core? Because the heater core will not get dirty, the evaporator core (A/C) is the one that gets all plugged up over time and kills your air flow.
You already know this but for other readers...
The inlet side of the evaporator core collects dust, pollen and moisture and over time turns into mold. The mold will eventually grow so heavy that air can no longer flow through the evaporator and results in low air volume from the air ducts.
All GM HVAC (heating, ventalation and air conditioning) systems push the inlet air past the A/C evaporator first and then past the heater core reguardless of the settings of the contols....hot, cold, defrost, vent, heat, floor, ect. It doesn't matter, the air always flows through the evaporator first. This is why the evaporator plugs and the heater core never does....unless your car did not come with A/C and we all know that is unheard of on a Bonneville.
A heater core can plug up internally and prevent engine coolant flow, but not externally to prevent air flow like an evaporator can.