Control what though the screen? If you hook anything up and want to control it via a switch, you will have to install your own switch... or you can likely buy the panel/harness and install it... hopefully it doesn't need the dealer to flash to make it work.
I just picked up my 3500 Longhorn with aux switches. Switches are integrated into the overall electronics. Configuration is done through the commercial menu on th dash. (Normal on, normal off, momentary, etc). On the drivers side firewall towards the top are two Deutsch connectors that are used with the wiring harnesses that are included.
It would appear to be a tough task to add this to a truck that didn’t have the setup from the factory.