Passanger side heat

I own a 99 Silverado. When heat is on the passanger side floor heat blows cold yet the drivers side is hot. No matter what setting seems to be the same. Inlaws own a 2000 Silverado and the same thing with theres. Any ideas on what could be causing this.

Thanks

Mine did the same thing.....2001 Silverado. Took it to the dealer and, if I remember correctly, there is a service bulliten for this condition. They said the fix was to remove the dash and re-do some seals against the firewall. The service guy then proceeded to tell me that since that took some time, what they've been doing was to stuff some insulation up against the firewall and any other place he could get at. He told me that seems to work pretty well.

So I went and got some of the round foam strips I've seen used to insulate around windows in homes (about 1" to 1 1/4" in diameter) and used that. Stuffed it everywhere I could up under the dash....anywhere I thought air might be coming in at. All the cracks and crevices.......even down along the sides under the kick panels.

She says her side is now warm as toast!

Just be sure not to cover up the heat outlets. Good luck!