I believe the problem is two fold, which gps 120 do you have, nema or seatalk?
If it is sea talk the platter does not power the sea talk connection, so you have to get 12 volts to the gps thru a junction box. If you are using a nema gps then you still need to get power to the gps.
I have an RL80C Plus, my simradfluxgate has gone south, I was wondering if I can connect the plotter via sea talk to the simradautopilot and use it to steer by cross track error rather than compass heading ?? the manuals seem to ignore that issue unless it's a ray auto pilot.