The clusters are only grounded through the mounting bolts. It is a wise idea to add a pigtail to a grounding point on the cluster long enough to access, and bolt it to the dash frame,/ column support
In the photos he has the cluster out of the dash when he is testing, my bet is he does not have a ground and is back feeding all kinds of circuits to light the dash lamps, hence the odd headlight switch behavior.
He is also probably back feeding to make the gauges function which could have disastrous effect on the gauges if the ground path is lost.