i have just encountered the same problem this evening. Went to change the bulb but noticed the one i was taking out is absolutely fine. Put in the other side which i know works and as i suspected the buld was fine. So I'm stumped at the moment. Will check fuses and rear lights tomorrow. If you do find out what the problem is or the garage sort I'd be greatful if you could pass the knowledge my way.....likewise if I work it out I'll do the same.