Dim display, presuming you have brighnes and contrast set in menu, note:
This is not always the display modue. You have series resistors in the power rail that can degrade, so before condeming the module, check that it is getting the proper voltage per service manual.
Not to hijack, but I've got an XG-75pe with a loose (but functional) channel knob. I'm familiar with Motorola gear, but have never worked on anything Harris yet. Have you done any knob repairs on them?
Thanks!
If the rotary knob is loose, check the spanner nut that holds it to the frame, they can work loose over time.
That not the case, the switch itself, may be failing and needs to be replaced.