There is No 5th Gen cabin air filters....
It wasnt even an "option"...
They are only available in 2000's and up...unfortunately!

 

Same here I don't have one on my 99 either, however I have a Pontiac Torrent 06 that does and the new 2013 Equinox has one also, Mini vans had them also I had a Transport Pontiac that had one. When your AC is not cooling real well this is something the shops will check.
I have also been into the dealership and have had a couple of friends who work in the parts department, and they have the ability to tell you what your vehicle has on it by the VIN Number. I new someone who bought a car used and just had the keys to open it and didn't know that a remote was available until they checked it out.