I've got a reg cab with no a/c, just a three position slider (vent/heat/defrost) and kick panel vents. The trouble I'm having is getting the damn things closed. No matter if I push the lever in or pull it out, they still seem to let cold air in. I've stuck my head under the dash and can see the flappers moving, but I can't tell if they are open or closed. Any ideas/suggestions?