Sorry, was trying to make time to do a write up about fixing issues with ex police commodores but this will have to do.
The steering wheel switches are fed from the dash board. Different switches present a different resistance to the dash board which then uses the resistance to work out which switch has been pressed. Police commodores tap into this circuit to allow the top right switch to operate their hands free system for the police radio. When they rip out their stuff before selling them they leave the return wire open circuit resulting in the switches not operating. In this mode, the dash board changes the display to show the speed while traveling and odometer when off.
To restore operation, you need to reconnect the orange wire with black trace back to the dashboard. Remove the bottom trim under the steering wheel.
In the image you can see the orange with black trace has been cut when they installed the police equipment. the orange / black trace wire goes off with a white wire and comes back with a blue wire. You just need to cut the blue and white wires and connect them together (on the orange with black trace side of the cut).
After that, your steering wheel buttons should work again.
You will also probably find that the front speakers don't work. You just need to reconnect the speaker wires where they attach to the radio in the centre console.