Cluster aint supposed to work on I mode, only II.
The needles going half way mean it's got a low battery. If you check diagnostics you will have DTC 9318! I'm sure of it
My Focus is the same, the lights dim like hell when I put my quickclear windscreen on and sometimes dim when I got the sub on loud. They even flicker when I got the windscreen wipers on. It's had a new battery about 12 month ago. Going to fit a second earth soon and hopefully that will fix it. Only thing left is the Altenator