those things are a pain in the butt.. just wait until the auto diming function doesn't work. This is my second mirror and it quit on me a year or two ago.
Here is what the owner's manual says:
Temperature Display
The temperature can be displayed by pressing the TEMP side of the switch. Pressing the TEMP switch once briefly, will toggle the temperature reading on and off. To alternate the temperature reading between Fahrenheit
and Celsius, press and hold the TEMP button for three seconds until the display blinks F and C. Press and release the TEMP switch to toggle between
the Fahrenheit and Celsius readings. After five seconds of inactivity, the display will return to a normal temperature reading. Press and release the TEMP switch to toggle the temperature display between Fahrenheit or Celsius. If an abnormal reading is displayed, please consult your dealer.
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2000 Silverado Z71, 5.3L, K&N GenII Air Intake, Y-pipe Exhaust (no muffler), Bosch Platnium +4 plugs, 242k miles and still running.