Where is the cab filter in an 2004 Chevy Classic?

The cabin air filter in a 2004 Chevy Silverado Classic (assuming you mean a Silverado, as there's no "Chevy Classic" model) is located behind the glove box. You'll need to remove the glove box to access it. There are usually screws or clips holding the glove box in place. Consult your owner's manual for the precise location and removal instructions, as there might be slight variations depending on trim level and options.