How do I access the hot/ cold flap or (damper) from under the glove box?

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I replaced the thermostat, flushed coolant, and drained the heater core. I know the unit reader for temperature gauge that displays on the dash does not work. It stays below C all the time. The coolant system is functioning correctly. I have found out by investigating that the hot and cold air both blow out air temperature equal to that of outside the truck. I believe I need to unclog the switch fan from behind the dash and replace temp reader guage