Garmin Map Pilot is provided on an SD-card. You're car will have an SD-Card slot somewhere, either in the Audio20 unit itself (older models) or around the centre console (newer models).
Pressing the NAVI button with bring-up the Garmin Map Pilot satnav system from the SD-Card on the Audio20 display.
The SD-Card contains both the navigation software and the maps data, so make sure you purchase the latest version.
Dealers charge £499 for a first-time SD-Card, or £99 for an Update SD-Card. The contents on either is exactly the same, and members on here reported that they were able to successfully use the £99 Update SD-Card in cars that never had Garmin Map Pilot before, but proceed at your own risk.
Also, the Garmin Map Pilot SD-Cards are available for less from eBay and Amazon, though some members here reported that the TomTom Live Traffic data wasn't working on the cards purchased online. So, again, proceed at your own risk.