Problem solved! In summary, the Fusion 700i has a black lead attached to the back which has a 8 pin male connector and a standard 5 pin NMEA
male connector at the other end. This does not connect to a T, but simply plugs into an inline 5 pin NMEA female connector (buried beneath the dashboard) which is at the end of a black/white dropcord which then goes to the T and eventually makes contact with the wired remote
When replacing the Fusion 700i with a 750i, the latter has a black lead wired straight into the unit with a standard NMEA 2000
5 pin male connector at the end. This connector needs to be plugged into the inline 5 pin female connector referred to above. You then select the 750i as the stereo on the remote
control and the remote will work.
To get to the elusive connector I had to cut through many cable ties so that I could move the things around and trace the lead from the stereo.
Thanks to all for their comments and suggestions.