Why would the digits fade out and blink in darkness on a KX-155?
Yes, there is a trim pot on the display circuit board that adjusts the dimmer level.
The display intensity pot is R438. The red arrow points to it:

The dimmer pre-set should be marked on the outside cover, and is adjustable without removing the covers, but it only adjusts the max brightness.
It could also be the photo cell that is bad or dirty. When the display starts to loose it's brightness, that's usually a sign that it's about to go south. It can be readjusted with the pot a couple of times, then is only visible when the radio has warmed up, then finito. Try a new photocell, very easy to replace.
Be sure to observe all regulations concerning avionics maintenance!!
Source of info: http://www.337skymaster.com/messages/showthread.php?t=478