They discontinued the cabin air filter for the Silverado starting on the 2003 models. there is a Technical Service Bulletin that came out to make us techs aware of this change:
#PIT3330C: Passenger Compartment Air Filter - keywords dirt dust HVAC case - (Nov 9, 2007)
|Subject:||Passenger Compartment Air Filter|
|Models:||2003-2008 Cadillac Escalade Models|
|2003-2008 Chevrolet Avalanche, Silverado, Silverado Classic, Suburban, Tahoe|
|2003-2008 GMC Sierra, Sierra Classic, Yukon, Yukon Denali, Yukon XL, Yukon Denali XL|
This PI was superceded to update Model Years. Please discard PIT3330C.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
A customer may inquire about the availability of the passenger compartment air filter.
Starting in the 2003 model year, the passenger compartment air filter is no longer available as a factory option. With the redesigned HVAC case, the opening for the filter cavity has been molded over.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.