I just had a quick look at the Spec's for the Garmin 220,
and it says there is NO Audio / headphone Line Out.
Under Bluetooth it says "Helmet or Headset use only",
which probably explains why it won't connect to the Trophy as a "Device".
Sorry to be the bearer of bad news, but looks like it is only the physical Triumph mounting bracket
that is compatible with mounting the Garmin 220, but not the audio or Bluetooth connections

Referring back to the advice that your dealer gave you :
"When ordered from Triumph dealership, they advised the unit was designed to work with Garmin zumo 220 or 660. I opted for the 220..."If they inferred that the 220 features such as Bluetooth and Audio out would work with the Trophy audio system,
then I would take the matter up with them, as such advice would have been wrong.
If you got the 220 from elsewhere, I would still speak to the dealer - to see what they might be prepared to do to help,
and to alert them about mis-advising any other future customers !
If from an alternative supplier, you could try and see if they would accept it back
in exchange for the 660 plus extra £££'s ? or maybe consider selling on E-Bay or similar ??
