I have a 1997 ford ranger XLT and there's something wrong with my heater?

When I turn the heat selector it doesn't do anything. Could there be something wrong with the vacuum. My truck is a 2.3 l 4 cylinder with 8 sparkplugs. How can I fix this? Also could something be preventing the trap doors from opening when I put it to a different selection

Hot all the time? Pretty common, Broken blend door