I am resurrecting this tread in hopes someone has learned something further... I am having the same issue in my '00 4Runner Limited.
I am sure the culprit is not a bad bulb, as I replaced all of them. Further, the lights for the entire radio flicker/dim all at once pointing more
toward a loose wire or a worn connection on the rear harness plug.
Does anyone know definitively which of those wires controls the radio illumination?
Seriously. Thirteen people above and there have been other threads about the same issue. All say replace it with aftermarket. It can't be fixed unless you are an electrical engineer. I linked the YouTube above on how to replace it. Twenty minutes if you have your tools organized.
My son has a 2000 Tacoma with the same radio and it still lights up. It's fun to have the cassette player mainly as a curiosity. He has to play his iPhone through one of those cassette adaptors. It sounds terrible.