Why air conditioning , right side vents, 1992 buick do not work?

Air conditiong, works all, except right side vents. Right side vents will blow heat not cool. 1992 Buick , LaSabreWhy air conditioning , right side vents, 1992 buick do not work?The blend doors are not working properly. It is probably controlled with vacuum and the vacuum hoses are dried out and cracked, it happens to a lot of cars as they age.