Hi and thanks in advance for viewing. I have just bought a 1997 silverado. I went to turn the ac on and noticed a ventilation problem. When I turn the blower on to the cab it blows out of the vents fine but when I push the AC button it switches to the floor and blows cold. When I turn the AC button off it switches back to the upper vents but has no cold air. can someone tell me how to correct this problem.