If you don't have a button, then there is a wire coming off the climate control that gets grounded to change the reading.
In my experience with the 1997 SSEi, GM changed the dashboard wiring harness. In all models, Canadian and US they had a wire from a passenger floorboard ground to the main connector of the dash. On US models there was a wire from that main connector to the HUD. On the Canandian models the wire didn't continue after the main connector. The grounding of the US version caused you to see MPH and the Canadian woudl see KPH.
Likely your wire is loose, not grounded or something else occurred. Having an FSM that showed the schematic would tell us exactly what is up wtih it.
00 Regal GS: All kinds of cool stuff.