I have a 2004 Avalanche and understand the passenger compartment air filter is located behind the map case (glove compartment). I removed the compartment, but was unable to locate an access slot or drop door. Any ideas of its location?

Welcome to the forum. My wife also has a '04 and it doesn't hae a cabin air filter. If you give your vin # to the dealer parts department they can tell you for sure if you have one or not.