Do a search on A/C contols (and related words). There is a lot of discussion about the vacuum system and how it can mess up the flow of air to various vents as well as the mix of air. You will find ways to try and ferret out the problem.
Also, there is a little valve on the firewall that controls flow of warm air. Can't remember the name but it can be easily repaired with inexpensive kit available at FastLane. A small rubber diaphram gets old and ruptures.