1998 GMC why does the rear view mirror temp work in cel. and not F. also dimming does not work?

I just had this same thing happen to me, i was quite depressed as i never knew how much i looked at it until it was gone. Out of desperation i held the Temp button in and counted to 15. i looked at the mirror and the Celsius light was blinking so i let go and hit the button again and the Fahrenheit light was blinking. After a second the temp was showing in Fahrenheit again. I hope this works for you. I just had this same thing happen to me, i was quite depressed as i never knew how much i looked at it until it was gone. Out of desperation i held the Temp button in and counted to 15. i looked at the mirror and the Celsius light was blinking so i let go and hit the button again and the Fahrenheit light was blinking. After a second the temp was showing in Fahrenheit again. I hope this works for you.