where would it be on a 99 k3500? I pulled the bottom panel under the glove box towards the center hump, but just saw the evaporator. Did not see where else it might be. not much room between the blower and the panel I removed for there to be a filter.

 

as fry i know that an't no cabin filter

 

I think you must be right. I can't see any place it could be. Seems strange for a "LS" model w/power seats, windows, etc. for there not to be a cabin filter to protect us poor drivers & passengers.