Through a 2 amp fuse, connect it to the cable leading to the fag lighter itself, its accessable just below the handbrake. There is a negative here too, or also some exposed chasis grounding points within the dash structure.
If you pull off the large piece of trim at the drivers left leg (the section between the carpet and dash) open the fusebox (at the drivers right knee) and pull the end cheek off the dash (the drivers side version of the panel that at the other side contains the airbag keyswitch) you will easily be able to suss it out.
You can but a trailing lead fag lighter, to attach to the wiring and plug the sat nav adaptor into this, there is space through the dash end cheek to install these together - tape them together in case they work loose.
You can easily then poke the sat nav plug out through the padding between the A pillar and the dash.
If youre not competent in attaching to the vehicle wiring, and using the right cable etc. speak to some one electrically qualified.
Please be careful to route the wiring securely away from the knee airbag!
There are also some adptors out there that take a lead straight off a device that fits directly into a spare fuse way - 2Amps is enough - bit I dont like these - and you still need to source a negative from the chassis.