Your AFM supplement doesn't need to be signed but should relate to an approved modification under which the equipment was installed or otherwise later inspected and certified against which will have shown compliance with indicator requirements and any associated annunciation.
Thanks Wigglyamp.
The FAA supplement relates only to the G430 in general and is not specific to my aircraft.
As I do wish to be compliant and fly enroute in controlled airspace how do I achieve the 'Minor Change' entry into the flight manual? Is it a case of presenting the aircraft to an approved installer and asking them to do it? If so that is where I am being told that I must upgrade the software to a 430W, install new cabling and a new aerial.