Pull the interior light bulbs, see if it still happens. If it does, or after a week or so of no results, pull out the radio and disconnect the wires from the back. Try it for about a week or so, too. If the fuse still blows with either of those disconnected, you have narrowed down your search to the central locking or whatever else runs on that fuse. Use the proper fuse rating, going too much higher could cause problems.