Just bought this rig. The cab ventilation does not work. I can hear the fan

blowing out the defrost and floor vents. Not the dash vents. Further symptom: the fan seems to run on all settings except high.

you have a vacuum leak under the hood.check the smaller vacuum lines at the firewall,passenger side first.start the truck,turn the a/c on high or medium,listen for hissing sounds.if needed run your hand around the vacuum hoses to find the leak,repair the hose.then after you repair the hose/hoses.when you have a vacuum leak,the blower will default to the defrost and floor vent MODE!check all settings on the blower after repairing,low,,medium,high.post findings!