Problem with the heater, vent, and defrost switch...please help

I have a 1994 Chevy Astro Van and I'm having a problem with my switch that controls the heater, vent, and defrost. Right now it is stuck on defrost it won't switch to vent or heater.
The other switches on the panel work fine, so what could this be? Where do I start? Please can anyone help me?
Thanks in advance!!

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