you may want to try and find some resistance in the circuit such as the metal on the taillights may need some cleaning with steel wool or something like that. I believe the brake light circuit warning is flashed because there is something in the circuit that is causing the bulbs not to get full power but they still will work. Just an annoyance for us. Then check the bulbs maybe change them, check the things that hold the bulbs in place and then if that doesn't work change the brake light switch.
<img src='http://www.bmw-forums.com/forum/uploads/photo-3078.jpg' border='0' alt='user posted image' />
'93 325i 5 Speed
-2 12" MTX subs
-16" Double Spoke Rims
-OEM Clear Corners and Tails