Could be something as simple as a ground or a bulb pushed in too far. I'd made sure all the fuses are ok, maybe swap that one fuse out anyways just to make sure it's not bad, then I'd remove all the affected bulbs and inspect them and their sockets. Maybe even a squirt of contact cleaner in each. (not eye contact cleaner silly )

If nothings changed then you figure out where the last place the power is, you never know...someone could have wired the ground or hot/power on for a speaker box and amp in the back to one of the taillights and screwed everything up. I've seen worse, haha.