I have a 2500 HD Duramax. The air conditioner does not blow as much air as it used to. I know it has a filter but where is it?

'02 Max, Chipped
'59 Apache, Stock
+ one very fast BMW

if is 03 and older u have cabin if it is newer u do not have them the cabin are under the glovebox there are to screws holding a cover up pull the cover down and your cabin will be behind and clip thing. there are 2 of them in there