The air conditioning filter on a 1998 Chevy Astro van is
not located in a traditional, easily accessible filter housing like many newer vehicles. The 1998 Astro doesn't have a replaceable cabin air filter in the sense most modern cars do. Instead, the system relies on a combination of the HVAC system's design and the heater core to filter some of the air. There's nothing you can readily replace or change to function as a standard cabin air filter.
To improve air quality, you might consider focusing on keeping the HVAC system clean overall (having the vents cleaned, etc.) and possibly investing in an aftermarket solution, though those are not readily available or designed for this model.
