Trailer Mode will not activate

[KV Bruce]

2005 2500 6.6 diesel, Allison Transmission.
Pressing the button on the shifter handle will not effect the trailer mode option or the no-overdrive option.
Am I missing something easy? Any help would be appreciated. Bruce

 

[Mooseman]

Maybe the switch? If you have a manual with schematics, see what wires send the signal and if the circuit closes when you press the switch.