Sounds like a problem with either a stuck blend door or a servo motor, you really need a pro to check this out as these are very hard to work on and can get really expensive very quickly. So check out reviews on shops in your area, ask friends and co workers where they have their cars worked on and make a decision. This vehicle is old enough that a dealer would be a waste of hard earned dollars, so a good independent shop would save you some money.
Actually I would take it to the dealership. This can be a complicated repair that must be done right.
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