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It sounds like the back light is no longer working well. I don't know it's a capacitor issue or not. I imagine that the CCFL is about to die or the high voltage drive circuit is not functioning well enough. I'd simply use LEDs replacing the CCFL.
Yes I think you are right. It seems to be a common problem with the 90s Toyotas and Lexus that have multi monitors. I started taking the monitor apart but it would be very difficult to get to the ccfl. Since this multi monitor is rare and expensive, I decided to stop as I didn't want to break the fragile wires. They are 27 years old! The monitor functions still work but you have to memorize where the menus are
Here is a picture of the backside of the monitor before I put everything back together