Where is the ac cabin filter located on a 1999 Yukon?

If it's like the pick ups it's on the passenger side . Lay on your back in the floorboard and lookup at the bottom of the hvac box, you should see a rectangle piece running front to back held on by a small screw . The pick ups have two filters in there, one will almost fall out , the other you will have to dig out. Make a hook out of a coat hanger to hook and slide it toward you.