On my '87 wagon, there is no air filter. Just the air duct that comes from the passenger side of the firewall area. Right below the windshield.
On my '96 GL, the filter is underneath the black plastic cover, passenger side of the firewall, right below the windshield.
In both cases, access is from the engine compartment. I looked in your profile, this is for your '91? I don't know if you'll find a filter or not. One other thing to check is that the drain from the evaporator housing is clear.
Where the 2 A/C lines go through the firewall on the passenger side, look below the bottom line and you'll see the short drain pipe for the condensate. It may be clogged with stuff, and the condensate will build up as standing water in your A/C duct.
You can use a coat hanger or whatever, to clean it out.
Good luck! And BTW: If the drain is clogged, and you jab something up there to unclog it, chances are good you're gonna have some real nasty water and stuff come puking out too.
