Where is a fan filtter located on a Kenworth W900 heat is working but not blowing out hard enough?

Your HVAC filter should be located under the hood, on the right side of the vehicle, and against the firewall. I believe the W900s all use the same cab filter as the T600 and T660, so it'll be encased in a plastic cover which is hinged at the top and held in at the bottom with a rubber strap. There's no need to replace it if it isn't damaged - it can simply be washed out and reused.