I had an 85 Mark VII...
When I purchased it dash was off.... Guy said it needed a new speedometer unit which he had...
I knew nothing about the car at that point...
When putting it all together I found that the entire dash unit needed to be plugged in, in order to get the dash to light up... This ment all units...
Message center, Speedo, etc.... the entire dash... Which tells me that there is a common ground...
What I believe is the Orange Wire - What I heard from Mark VII owners back when I was working on my 85...
But first look for the fuses... Then Find Old School.... He's a god of Mark VII's
Good Luck...
Oh and :W to :V