2005 GEM Switch issue?

2005 GEM, following brief period of non-use I’m having an issue with the electrical system I can’t shag down. Batteries are fully charged and the charging system works fine. When I turn on the power then turn the key, dash lights up and I hear a click, then it starts clicking more with display fading on and off. Eventually it will stay on but any use of electrical system, (turn signals, lights, etc) it goes back to fading in and out with clicking sound. 10A fuse appears good, I thought it was the key switch but after removing it, it appears OK. Any advise would be greatly appreciated.

Check all battery connections. Might also be a terminal corrosion issue.