The bulb for both the backlighting and the indicator light were not working on my A/C button and the backlighting was not working on rear defroster although the rear defroster indicator still worked. I purchased some led T3 bulbs and replaced all four bulbs. The backlighting now works for both now neither indicator light works. Any thoughts?

 

It is probably the bulbs itself. I bought some time ago several cheap T3 bulbs.
Just check everytime after replacing a bulb on one button if the button light is working. If not, replace it.
In my case the 3rd one I replaced was the one that started working allright.