99 GMC Sierra 1500 - top dash and floor vents inoperable

You only have one fan motor. It sounds like either the small motor that moves the flapper door (air valve door) that changes your vent positions inside the duct work has failed or is stuck. When you change vent positions from heater (foot level) to dash to defroster do you hear any humming or soft buzzing from under the dash that tells you the motor is working or trying to change the air valve position but is stuck?

Cheers - Jonathan

Thanks, Jonathan for your reply. I'll take a listen under the dash. I haven't heard any "noticeable" humming sounds when I change selections from heater to defroster, but I'll take a closer listen... with my luck, the flapper motor has failed.

Thanks again, -pup

With luck the motor isn't burned out, just the flapper door is stuck in one position from some gunk. If that's the case you may be able to just get in far enough to reach the door seals and clean them off.

Cheers - Jonathan