only some vents blow on heater and air conditioner on 2000 montana

[99stockgtp]

Only defroster and floor vents work with heat on and only defrost vent with air on. I do know that the heat takes forever to warm up and is probably due to a bad thermostat. If its the vent door actuator how do I change it. Would that even be the problem? Thanks in advance for any and all replies

 

[Steelcity]

Its real easy to see up in there and watch the diverters and linkage move with the selector . Just lay on your back and look up towards the middle.