well you have established it is not the st6000,so it is either the wiring
or the course computer.
i would check the seatalk
connection for voltage before suspecting the course computer.
once you have exhausted that possibility and you still have an error then think about replacing the type 300(worth checking ebay for these old units,about £300 for an unused one on ebay)
as a test ,run 3 wires direct from the 300 to the st 6000,bypassing the old wiring
,just get the colour coding right,if you dont have some small spade terminals you can push the wire ends over the red ,grey and yellow terminals on the st6000,the 300has terminal block connections.