I had the same problem with mine on a 2000 WJ (found JF when searching for solution) and ended up ordering a new brake light from somewhere in California. You can search and find where people use sandpaper and other methods for cleaning the contacts, but none of that ever worked for me. What would work for a week or two was simply to pulled the socket with light bulb from either the backup light plug or the turn signal. However, eventually it would do the same thing.
Hope your fix works for you.