A/C UNIT ON 2004 Chevy Silverado Z-71

I HAVE A 2004 CHEVY SILVERADO Z-71 WITH APX 50,000 MILES. IT HAS A BI-LEVEL A/C UNIT. WHEN YOU CUT THE AIR ON A/C ON THE DRIVER SIDE, IT WORKS LIKE NORMAL. BUT ON THE PASSAGERS SIDE IT ONLY WILL BLOW OUT HOT AIR.(EVEN WITH IT ON 60 DEGREES). ANYTHING I DO WILL NOT CHANGE IT FROM BLOWING HOT AIR ONLY ON THE RIGHT SIDE. PLEASE ANYONE GIVE ME IDEA OF WHAT COULD BE WRONG. THANKS SNOMAN