Cabin smell

No filter in there.

From GM

Condition/Concern:
A customer may inquire about the availability of the passenger compartment air filter.

Recommendation/Instructions:
Starting in 2003 model year, the passenger compartment air filter is no longer available as a factory option. With the redesigned HVAC case, there is no longer a cavity for the filter. The GM parts catalog indicates there is a replacement filter available; however, there is no provision for it.

Most parts stores sell a spray that you can shoot into the system that kills any bacteria or mold that may be in there without damaging the heater or AC cores.
Check that the system is draining properly, a plugged drain (located under the front side of the firewall) can keep water inside the system.

Another tip is to use a spray bottle and rubbing alcohol to spray into the recirculation door and then let it set a while to kill the crud.
If it's a hot/humid day and you have been using the AC a lot, turn it off from AC and let the system dry out. Takes about five to ten minutes normally.