What did catch my attention. If the head unit is otherwise properly grounded when installed. I would take in my case a wire with a couple of alligator clips and run a ground from the metal plate back to a real ground. There is a slight chance the head unit is not being properly but only partially grounded in your video.

Or just take you meter and read ohmage from that metal plate back to a good ground. It should look like a short circuit. Or really high resistance. A semi ground has to be exiting currently or there would be no function at all. This is a very long shot as the two wire plug that is installed looks like it has a ground. Plus a power feed.