Reality, Logic, Factory Service Manual
I own the 1994 FSM and have looked at the 96 FSM. The diagrams are also available on alldata, on goldsswagon.com, (many thanks for his great site), and in libraries. With the exception of the 96 Impala they seem the same.
I have found no "extra" wires in the two plugs to the cluster. I have had a oscilloscope on the Vss line and it's signal is one way and matches what many others have said about the pattern. I always take the connectors with the cluster and none have mystery wires. I have traced the circuit board between the speedometer chip and the cluster plugs. Everything matched my FSM. No one has ever reported extra wires connected to a stock PCM.
There are some good unused pins on the cluster. I think to meet European law they added a separate light for the parking brake warning and a light for the Hazard flasher. This forum has a long history (with gaps from the crash) but the general agreement is only speedometer repair places can change the odometer in the cluster. A internet search will show a few offering to "correct" the mileage of a cluster when they fix or replace the cluster. How can they do this if the mileage is stored "somewhere else".
For those who think the Odometer information is "somewhere else" than the speedometer chip on the cluster please supply diagrams, pictures and documentation.
I think the video is real. Of the many speedometer/odometer failures I have see my own first car would display two different odometer readings. I did not try to repair this one as I suspect a damaged "chip".
For the more common failures of dim outs and blank outs the forum has two repairs. See "1995 9C1 Intermittent Dash Display Fix" for both with great pictures and instructions.