Great post/info.
I am at that point myself with my STX. Changed the cluster to FX4 and took it to the dealer. They said they would try it for 85 dollars without garantee. Kinda made me nervous. Next I will tell them to send "As-built" data to cluster. This post has helped.
About the compass wiring: I attempted to install this also, but have been unsuccessful. The compass (4 pin) wiring routes to the passenger side above the fuse panel. There is a connector to send these wires to under the hood. PINs are:
Pin 1 White/orange strip (compass data +)
Pin 2 White/Black Strip (compass data -)
Pin 3 White/lite blue strip ( 12 Voltages supplied in Start and Run mode)
Pin 4 Black/lite blue strip ( ground)
Mirror wiring routed to drivers side (6 pin):
Pin 1 White/lite blue strip (12 volts at start/Run)
Pin 2 Black (ground)
Pin 3 Black/Pink strip (Trailer Tow Relay Reversing lamp switched power)
hope this helps someone