You used to be able to get 'w' shaped pieces of thin spring wire that slid inside the fender and kept the wire in place. I'm sure those are basically unobtainable now. If you want a clean run for the wire on the rest of the bike, you can use a shot of compressed air to blow the wire down a length of brake housing, and secure it to the down tube with a set of brake cable clamps. It looks like it belongs and helps keep everything in place.