I would concentrate my troubleshooting in the one that we scroll at the left side of the steering wheel, I don't know how it is called. But my basic knowledge call that a dimmer..a rheostat..or a potentiometer. Its controlling the flow of current towards those display lights as you can observe while scrolling it. That type of component gets dirty, changes in temp, and humidity plus molds makes the contact bad and maybe the reason for the insufficient current and voltage supplied to those bulbs in the displays causing it to glow dim.
If its a single bulb thats busted or gets dim, then its the bulb. but if all of them in a module or a cluster gets dim, then one part incharge of the supply messes it. Just like an old stereo, when you first turn it on at medium volume, it just whispher, minutes later , it shouts..My "don't do this at home" remedy is to flush the volume with lighter fluid(it dries fast) to remove those dirt and molds. The volume is the same to the Dimmer in the 4runner.
So I'll do it if the bulbs in the cluster of my 4runner glows abnormally or I'll just replace that thing. The one we scroll may have a belt to the real component that is mounted somewhere.
It just my opinion and may not be correct and still would suggest the dealer to look into it and have OEM replacement..
..the center aircon assy display if thats what you mean is also contolled by that dimmer while the stereo is independent. Try scrolling it up and down(fast) to loosen the dirt and enhances the contacts if it solves your problem.
