The steering wheel controls connect directly to the instrument cluster, and from there, the whole shebang is broadcast over the SW Can.
(I.E. the cluster reads the cars speed, engine RPM, engine temp, Ignition switch status, and lots of other little things from the SW Can),
and will broadcast any steering wheel switch button presses over the SW Can for the Radio/Sat Nav to pick up and use....
Sounds to me like its one of three things...
1) Worse case : Your Cluster is fried.... (I dont really think this is the case, as they are hardy little things, and dont die easily)
2) Second Worst Case : The SW Can bus wiring is shorting out somewhere....
Best way to test this is to check to see if the HVAC functions work correctly. If the SW Can wiring is fried, then you will not be able to set the temperature or fan speed....
3) Best Case : The Instrument Cluster plug is loose. (They have a lever latch on the electrical connector, and the cluster will work fine even when this latch is not in the closed position, however if this is the case, the plug can become loose, and come out....
If I was a betting man, I would be thinking Case 3, as this is an Ex Cop car, and highly possible that trim pieces have been removed to fit / remove the police accessories...
It is very possible that this connector wasnt fitted 100%...
Hope this helps....