Just done it at the weekend, piece of cake.
I used a blade connector into a spare slot in fuse box (switched live) to an inline fuse holder (Maplins) for live. Soldered into a female 12v socket (Maplins) earthed to metal cage behind fuse box (soldering one end to 12v female socket and crimping ring connector to other end)
Positioned satnav next to rear view ran power lead in roof lining, down pillar trim (hidden inside) and into fuse box where I plug sat nav plug into my new 12v socket hidden in fusebox
Cost a tenner max for bits
hope that makes sense to you helps,