2000 GMC Sierra Extended Cab K2500
Just had the same problem. All web searches on the problem said the problem was (1) an air bubble in the Heater core that needed to be purged or (2) the temperature control door actuator needed to be replaced.
While watching a few videos on the subject I noticed this one off to the side https://www.youtube.com/watch?v=Mp45ToY4Pbw
My owners manual detailed the "cabin air filter" but, I asked several factory mechanics about them and was told that the filter was an option that was not included in most builds. Since I didn't see it on my factory list of options I figured I did not have it.
After watching the linked video I thought that I would investigate further and found that My truck did infact have the cabin air filters. I picked up some Fram Replacements http://www.amazon.com/FCF8791A-Fresh.../dp/B0010DW5ZO
and swapped them out.
After 13 years of service the originals were PACKED with dirt and dust. I vacuumed all the residual garbage from the housing and now LOTS of warm air actually flows from the vents ! Total time for cleaning and swap was 20 minutes.