Front blower only operates in defroster mode

The air flow vents, which direct air flow direction, are actuated by engine vacuum. My guess is there is a vacuum leak that is causing the system to stay in the default mode of defrost. What you need to do is check under the dash for damaged or disconnected vacuum hoses as well as checking the vacuum supply from the engine.

When checking, start the engine and listen for a hissing sound which could help you locate where the leak is located.