The GHC20 communicates with the CCU (the thing you are looking for) over the NMEA2k network.
Your GHC20 is connected to 2k because it has power which it gets from the 2k wire.
The CCU should be connected to the 2k network. Its possible it is disconnected, causing the problem.
I would doublecheck all of your 2k connections
Most Garmin chartplotters can also operate the Autopilot CCU thru the NMEA2k network.
I'm just wondering, as a troubleshooting step, if you have tried using a garmin chartplotter, if you have one.
If your chartplotter detects it on the network you will have menus for Autopilot.
The CCU also connects to the Smartpump. That cable has some pigtail wires for the alarm buzzer (a small black buzzer) and the Shadowdrive.
Have you heard the alarm? Have you seen it and can you trace that wire to the CCU? They would normally have ziptied that near the GHC20.