I want to pick up a "live with ignition on" supply from the light unit in the headlining. Company car so needs to be removable - with no obvious signs of it being done afterwards. How easy is this to do?

Don't particularly want to run satellite navigation from the lighter socket.

Don't have the car yet so can't take it apart to look.

when installing a bluetooth kit, I found the easiest option, to geeting an ignition live, was to run a wire to the fuse box and tap into the connection that is marked with a phone symbol.

Fuse box would be the best place. It's been covered a few times so it'll crop up if you search.