Left side behind black plastic panel that is in front of the pannier you will find either 2 auxiliary sockets or the plugs for them Use them then you don;t need to take the Plastics / tank off at all
Think it's red and white permanent positive probably better to fuse with a 5amp and a brown the Brown = negative
You don't say what bike you have. On my 1200 the battery is under the seat and very accessible.
Also on the 1200 theres a satnav connector under the headlight purpose made for powering your chosen navigator. You can buy the connector's mating half at any Motorrad dealer.
Actually there should be an orange or a blue connector block (sorry rusty on the colour just at moment) up there by the radio box with an ignition live on it!