A tail light bulb socket might have moister in it or the bulb could be shorted out so check all the sockets and bulbs bec you have a shop.It also could be due to someone adding something to that circuit and it's over loading the fuse or a wiring harness going to one of the bulbs has frayed or melted or damaged wires.
Hope that helps and best of luck.By the way if it's an older vehicle you maybe to go to a U pull auto wreckers for the sockets but you should be able to get new replacements from the part stores or dealerships too.