88 Chevy cheyenne

Air flow will not change from dash vents to defrost to floor oulet.
Flow is just stuck on floor vents.

What might be the problem???

possibly the vacuum line going to the actuator. On my 86, it's a very small plastic, not rubber, tube. It's OD is the same as the regular sized vacuum lines ID. It plugs in to the vacuum system near the brake booster and goes through the firewall.