Low air flow from center dash air vents

While air flow is normal from side, foot and defrost air vents, very little flow comes from the two center vents (but some faint air flow is noted). Same problem with heater and air conditioning. The control knob will correctly divert air to the foot and defrost vents, but makes no difference regarding the airflow from the center dash vents. The control cable appears to be working normally (no slippage or binding throughout the full range). Am considering replacing the filters, but everything I have read so far seems to indicate that if the filters are blocked, air flow should be affected through all the vents, not just the two center dash vents. The two center dash vents appear isolated from each other, but must have a common duct somewhere, and I suspect around that point may have a common louver or flap, but I can't locate a schematic to know for sure. Am concerned about removing the dash because of the passenger side air bag is in the top of the dash. Please advise how I might correct this problem myself (am retired on low income). Your help is very much appreciated. The car is a 1999 Chevy Prizm, manual transmission, 4 cylinder, 5-speed, with heating and air conditioning. Thank you!