I had this problem not long ago and it was NOT a tail light There is a sensor for that. This has to do with the sensor that checks the brake fluid level and if the level is down it will behave with a light and beep. When the level is OK, as in my case, the level uses a REED switch that may malfunction and give you the same result. You can replace the sensor unit but what I did I took it out and while having it upside down I screwed, very carefully, a small sheet metal screw so as to keep the float in its highes position and... Viola... problem solved. I will look later on for an used unit to replace that one. For many years we did not have this kind of things in cars so I do not worry about it too much.
