The autopilot course computer is a box below decks. It gets heading information from the flux gate compass
. If that works then you have almost everything you need. The E120 usually talks to the course computer via Seatalk. The Seatalk network is a simple 3-wire cable. Look at your course computer manual and determine how many Seatalk connections it has. If it has only one then you will share that between the E120 and the ST6001 control head. Read the manual on the course computer as it requires some special considerations. If the computer has 2 Seatalk connections then one can be used by the E120 and the other by the ST6001. Also, you will likely need GPS
data on Seatalk. You may already have that. How does your E120 get GPS
data now? If it is by Seatalk then nothing more is required.
You will need to research
your boat to figure out how Seatalk and NMEA
are routed to various devices. It's very simple but not having a sketch of the system makes it hard to recommend exactly what you should do.