The two E120
displays are networked w/ SeaTalk hs.
Check the connection block w/ the terminators and check the rear of the E120 for the SeaTalk2 connection.
When I swapped my electronics I created a schematic
to help debug. Also, the factory wiring diagram had many deficiencies that now have annotations on my hard-copy.
Also both ends of every wire are labeled, 'cause wires tend to look the same after installation.
Wiring / electronic problems can be a pain-in-the-butt. Good luck!