1998 Ford Explorer A/C Maintenance

1998 Ford Explorer A/C Maintenance: My 1998 ford explorer 4WD 4.0 6cyl SOHC air conditioning system has dirty air coming out of it. Is there any filter that I can replace or anything that can be done to fix this?

Can you describe the "dirty air?" Does it just have an odor, or is it physically dirty? Does it do it on "Norm" as well as "Max A/C / Recirc" settings?

I don't believe there are any cabin air filters on the '98 Explorer.

-Rod