AC is great but no temp control.

I have a 99 Montana. The heater worked great all winter, and the AC is working great, however, had to warm the AC last week as we were on vacation, only to find that the front vents will no longer blow warm, or hot air. The rear heat/ac works just great.. Any ideas as to where to start looking?

Thanks

Anyone have any idea what I can do to fix this issue?