Hello I'm am new to this forum and wanted to share some input. My 1999 4Runner Limited radio backlights weren't working, but my LCD display backlights were working normal. After reading this forum and others about similar problems with the radio backlights, I decided to take my radio apart. Discovered that none of the bulbs were burnt out (5 bulbs) they were not getting power from the radio circuitry. Unfortunately without a schematic of the radio itself I couldn't figure out what actually powers the 5 backlight bulbs. Something in the circuit card failed. So now what. Since the LCD backlights work normal I jumped the power from one of the LCD backlight bulbs to one of the other 5 backlight bulbs. I confirmed the power wire with a voltmeter with the radio face removed and the circuit card plugged in to the radio. I placed the ground lead to the radio chassis and the power lead to the LCD bulb and that told me which wire is power (approx 6.5 volts). I then used a small jumper (wire) and jumped from the LCD bulb power wire to one of the 5 backlight bulbs. If you touch the wrong side of the bulb ( meaning ground) with power from the LCD bulb, it will probably spark meaning you touch ground. So then touch the other side and voila the 5 backlights will turn on! I used power from the left LCD display bulb and jumped to the closest backlight bulb, which was about 1 inch away. I then soldered 1 jumper in place. The backlights are in series/parallel so you only need one jumper. So now my radio backlights come on when the radio is on. Before they would only work when the exterior lights were on. Who cares, my lights work beautifully and no more fumbling with the radio in the dark! They will also dim with the light rheostat. When car is turned off all lights go off as usual.
This is what I did to solve my problem and I am not telling anyone to do this. You can buy a new or used radio too. Anyway lots of information here and hopefully you can understand what I wrote. Take care.