Hi Alan,
I don't think too many GA aircraft are fitted with squat switches linked to the undercarriage to mute the ADSB on the ground
Nowadays they don't need to be physical squat switches but simple options in an EFIS. They don't mute the ADSB but switch the Mode S transponder between ground and airbourne mode (having the effect of turning the ADSB on and off).
As an example, my EFIS has an 'air switch' where I've told it to trigger as 'flying' once the RPM are > 4000 AND the IAS > 40kts. It will switch the transponder 'on' once those parameters have been met and will then turn it 'off' once the speed drops < 40kts (for about 15 continuous seconds, I think). This way it doesn't think it's on the ground if I'm stalling etc. Clearly, this needs to be bypassed if you want to test the transponder/ADSB on the ground. I only offer it as a suggestion in case Chris' machine has such a 'switch' fitted, to save disappearing down other, more winding, rabbit holes.
Many, many light aircraft have facilities such as these now and it is beyond the preserve of Boeing and Airbus