odd tailight issue??

In the last several weeks I had two people tell me my left brake light was out. The first time I drove it home and checked all the lights and they were good. Yesterday at work same thing. I checked them and sure enough the left brake light was not working. Turned on the taillights and the brake light started working. Turned off the tailights and the brake light still works?? What would cause the brake light not to work until the taillights are on and then cause it to work for sometime until it decides not to work? A intermittent ground problem?