heater vent no work?

hey guys i bought a 1988 gmc sierra 1500 reg cab long box over a year ago now ever since i bought it .. whenever i put the heat on it only comes out of the top vent not the middle or sides now i found this kinda retarted lol and i was wondering if anyone had this problem before and if theres any kind things i should be looking for to fix this problem....

The is an actuator that moves the mode door (which controls where the air goes). Over the years, it can wears out and becomes too weak to move the door.

It is located above the tranny hump on the drivers side. The mode actuator is available thur AC Delco parts stores so you do not have to get it from a dealer.

//2000CAYukon