Passenger Toes Not Getting Heat

[Prathbun]

I have a '00 Silverado 2wd, 80K, 3 door cab and when I rode in the passenger side front seat during a recent cold spell, My right foot was almost frozen. I reached up under the dash near the kick panel and it felt like there was a hole in the firewall letting in cold air. I'll be changing the cabin filter today but other than that I have not done any further investigations. Any ideas??

Thanks!!!