Most MB's already have a screen antenna, integral. Most likely rear screen but Vi's are certainly windscreen, and are suseptable to RF interference from any USB kit.
You would be better sourcing a 'Fakra' adapter from car to HU. Be sure to get your male / female right when ordering. Ebay shouldn't be more than a couple of quids.
The blue (on the HU) wants connecting to the output that provides +ve 12v when the unit is on, blue there is possible too. It powers the antenna amp. If not connected you will suffer poor reception.
Brown (on the motor) will be the -ve, black will be something else. But confirm with a meter.
The canbus isn't usually an issue, on earlier motors at least. Although there is supposedly difference between hi and lo. Even on a 2003 model I would expect the canbus to be available in the motors connector.