My A/C blows cold or hot, no problem there and the blower blows whatever speed I want, but the switch selecting where I want it to blow is not working. I put it on the heater vents but it blows thru the defroster vents. I put it on the A/V vents but it blows thru the defroster vents.

Whenever I turn on the A/C system it will momentarily blow thru where ever the switch is pointed to, but after several seconds its goes to the fedroster vents. Help.