2002 jeep grand cherokee overland tail lights quit working, not fuse or bulbs.?

[jesse1987@ymailecom]

Front lights work as well as break lights and headlights.
Changed bulbs and checked fuse, neither helped. I'm calling tomorrow to have it put in shop, just wondering what it could possibley be as I'm hoping it'll be a simple low cost problem. Thanks a lot in advance.