You should do a Google search, there are a few pommy sites that have some good gear and information on Garmin products.
Other options are to wire the 12v power cable into the back of your lighter/usb plug, depending on the type used most can be done easily with piggy back terminals, you could also put a lighter type plug on the 12v power cable and just plug into your existing system, with either of these you will need to find a place for all the extra cable.
The another option is do it properly through a switched wire or as I have done back to the battery, as they run on 5v these things don't draw much power but a good robust 12v power system will never let you down.
Some of the 19 rides in my shed
2014 V2, Snoopy
2009 Wee, Pumbaa the pig
WR450F, The Blue Postie Bike
YZ250N smoker with rego, Stinky.
Yamaha MT09 (FZ09), The Scud missile.
Club Lead not Club Med.
He with the most toys wins.
Out of my depth in a puddle.
Live life on the edge you will see more that way.
Ridding a motorcycle keeps things in balance.
At the end of each trail and at the end of each day history is made.