You need to determine what all is on the same circuit, it's likely more than just your tail lights. Pull the fuse or while it's blown check to see what else is not working either. Only then can you begin a methodical search for the problem. Although a bare/rubbed wire is one possibility, it's only one of many. Besides a dead short in the wiring or in a failed component, you're also looking for excessive resistance overloading the circuit. This can also be caused by a bad component, a poor connection somewhere, etc. Have you checked all of your tail light sockets for moisture and/or corrosion?