[Richard]

Good find. You are not the first person to misidentify the marker light post as a ground. Many years ago when installing headlight relays, I used the marker light post as the ground for the relay only to experience unexpected results when turning on the headlights. The relay wouldn’t close when using the headlight switch, but if I turned on the marker lights the headlights would come on. I am not as smart as you and it took days to figure out that post was not a ground.