I've just fitted a permanent power supply for my sat nav, to stop having to plug it in to the cigarette lighter.
One tip here was very helpful, don't cut off the plug and simply wire in 12V, as the plug transforms 12V to 5V which the sat nav uses. 12V direct would likely blow it.
What I couldn't find out here was which wires to use (I have wired it to the under steering column fuse box). It turns out is was very simple - open and lower the fuse cover, undo two crosspoint screws, gently pull down the fuse box itself and you see together one red and one black wire, about middle top of fuse box. The black is earth, the red is permanent live. I cut those and rejoined them wiring in a cigarette lighter socket (cheap on ebay) and plugged in the sat nav socket to that. The socket and plug and spare wiring then fit neatly in the cavity behind and below the fuse box. I also have an inline fuse in the socket supply. Highly recommended to disconnect battery before doing this but I couldn't be arsed.
The sat nav itself fits neatly under the dash cover, just below the car information screen. It still sees satellites through the vinyl coated foam dash cover.