Air not coming out of vents

I have a 1999 Silverado 1500 .When you turn on the blower for the AC or the heater you can hear the blower running but no air comes out of the vents. Is there a relay or sensor that could be bad? Air does come out of the top vents when defrost is on. Air comes out of bottom vent also. 10 amp fuse for ac/heater is good.

check your heater-ac fuse 10 amp in the left side fuse panel.

the mode door motor is on the left side of your heater box. can you hear it come on when you change the mode from vent ot heat or defrost?

do you have air flow out of any vent when the blower is on high? do you have air coming out of the top vent when you have it on defrost? do you have air flowing out of the floor vent when you put it on heat?

do you feel a temperature change when you go from all the way hot to all the way cold?

Need more information...

you could have a bad mode door, door actuator, no power the the actuators, blocked incoming passage,