Hi, you must be new here!
So, GM deleted the cabin air filter for the GMT900. It might have been optional in the GMT800. Anyway, the slot for the filter is still there and, instead of an access door, the slot is just sealed off. You can cut out the rectangular portion sealing it off and install the filter. A retrofit kit will have a closure piece (the "access door") that is essentially identical to the factory piece. The boss for the screw to secure the closure is even still there in the HVAC housing.
Take a couple Advils and/or Ativans and slide up under the passenger floorboard. The HVAC cover is secured by three screws (7mm, IIRC). The rightmost and center are easy to remove. There's a third tucked up under the dash, in front of the center console- "good luck", you'll figure it out. Remove that cover and you'll see everything I pictured. I bought
the cheapest filter retrofit kit Amazon had. No complaints at all with what I received. I don't see any functional difference in this $13 kit versus the $40 Dorman kit. Cutting out the door is a hassle. But the real chore is cleaning the crud out of the fins of the evaporator without shoving it deeper into the fins. You can only access on side of it and you're working through a ~1" x 8" slot, in the footwell up near the firewall. Ideally, you can find something the same height as the floorboard (bucket, ice chest, etc.) to place next to the door sill, sit on it and lean backwards into the floorboard to work. I've been tempted to remove the passenger seat. But I gotta suffer the hard way first.
