I just use the 12v power socket to charge my TomTom V5 getting the correct plug from Buybits
Fused & Adjustable Hella / DIN or Standard Cigarette Lighter Power Plug (sku 10211)
You can do an easy, quick and dirty wiring job by splicing into the numberplate light cable that runs next to the underseat storage tray. 12v and ingnition controlled, splice in with a good quality bullet twin connector.
Make sure it's got an inline fuse, this is my prefered option rather than a direct to battery connection.