2000 Daewoo Lanos heating

Check for a vacuum leak under the hood first, you should be able to trace the vacuum from the firewall back to its supply, usually at the intake manifold. If you can't find a leak under the hood, look under the dash, the problem maybe that the vacuum supply hose to your system maybe leaking or unplugged to the component that is switching the door. If its not leaking, disconnect the line and suck on it to make sure the diaphram in the component is not leaking, it should hold the suction, if it doesn't hold replace the component