How do I access the heater control valve on a 1996 Toyota Tercel?

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I cannot control the heater temperature. I believe the cable end has come off the control valve lever. It appears to me it may be behind a plastic plate in the top-center of the dash. I tried prying the plate but it did not seem to want to budge. I don't want to break it in any way... How do I get it out of the dash if that is what I need to do to access the heater control valve.