cabin air filter 96

Where is the cabin air filter located for the upper vents in a 96 suburban k1500? I have very low pressure in my upper vents, any suggestions?

Don't think they started cabin air filters until around 2000. I know my '98 Sub doesn't have one.

Is there a way to increase the air pressure coming out of the vents?

Can you feel the air blowing out on your hand or face?
Is the blower running good on all speeds?
Is the heater controls switching to floor and defrost and dash vent?
Is the AC cold?
Got good vacuum from engine to heater controls?

Don't think they started cabin air filters until around 2000. I know my '98 Sub doesn't have one.
According to this link:http://www.drivewire.com/chevyparts/catalog/chevysuburbancabinairfilter.html they didn't start until 2000.