2000 Blazer. When floor (heater) is selected it blows on the windshield (defrost). Split, floor and defrost, only blows on the windshield. Split, vent and floor, only blows out the vents. All other modes function normally. I have disassembled and cleaned the selector switch (which controls and directs vacuum) no help, still can not get air blowing on the floor. Thanks in advance for any help with this matter, Don.