I've had a lot of car but this is annoying. All function buttons still work if you press them enough times, LOL.
Does anyone have a fix? It only happens on hot days and the hotter the less the digits are visible.

 

Sorry to say that the only real fix is replacement. Whether it's replacement of the screen and buttons, or replacement of the entire radio.

As for future prevention, do you use a windshield heat reflector on hot/sunny days? If not, you should. Keeping direct and excessive heat out of the dash will prolong the life of everything inside.

 

I have my stock head unit from my '08 Sport if you need one. Pulled it around 10K to install a Pioneer unit. I believe I still have the little card with the radio code too. Very little use, display nice and bright with no fading. Let me know if you're interested.

 

On mine the screen starts separating (I have a bright bar across display); it's such a nice theft prevention feature, better than all radio protection codes ; I will smash it with hammer and make a claim to insurance company
PS on my 79 Supra the aftermarket radio /cassette tend to switch stations while working and someone stole it . Sure he was very happy!