Air not blowing through front vents

It sounds like you heater core is begining to leak. Symtoms would include a coolant smell in the passinger compartment, loss of coolant and eventually coolant pooling on the front passingers side floor.
The other possibility is that you have mold growing on the a/c evap core. This would give a foul musty smell. The smell is completely different and this would cause no loss of coolant. For this there are anti-fungal sprays you can shoot into the system or just try some spray lysol.
IF it is your heater core here is the test & R&R proceedure.

Heater Core
Removal & Installation
1997-2002
To Remove:

NOTE: Heater core must be vacuum leak tested before it is remove from vehicle if a heater core leak is suspected.

Drain engine coolant.
Remove instrument panel.
For 5.4L (4V), if quipped, remove junction splash shield.
For 5.4L (4V), if quipped, disconnect starter relay cable.

For 5.4L (4V), if quipped, remove junction block bracket.
Clamp inlet and outlet heater hoses.
Disconnect heater hose from heater core by gently pressing it toward heater core.
Remove A/C plenum demister adapter.
Disconnect vacuum line.
Remove 13 plenum chamber top screws.
Remove blend door assembly from case.
Remove heater core.

To Install:

NOTE: If needed, clean and lubricate coolant hoses with plain water.
Install heater core.
Install blend door assembly to case.
Install 13 plenum chamber top screws.
Connect vacuum line.
Install A/C plenum demister adapter.
Connect heater hose to heater core.
Release all clamps from inlet and outlet heater hoses.
For 5.4L (4V), if quipped, install junction block bracket.
For 5.4L (4V), if quipped, connect starter relay cable.
For 5.4L (4V), if quipped, install junction splash shield.
Install instrument panel.
Refill engine coolant.

***Please note ***The instructions do say to remove the instrument panel (or dash pad assembly) this takes time and above average mechanical ability.