It's only on when the key is in the ignition, not all the time. Check the cigar lighter plug on the satnav cable first to see if that has a fuse in it, some do.
If the fuse really has gone then it is most probably behind the drop-down panel below / right of the steering wheel. There should be spares of each rating there and also and some grippy things for getting hold of the fuse to pull it out. If it's not there it will be in the fuse box on the battery or even in the boot under the panel marked, wait for it, 'fuse'.
The manual should tell you what rating the fuse is and where it is. Push comes to shove, pull them out one at a time and look at them - a blown fuse has an obvious gap in the conductor. get new fuses at Halfords and the like.