it's most likely the "dual-diversity" antenna lead. Check the perimeter of your front windscreen and see if you have a wire imbedded in it. Most U.S. BMWs after about 1993 had it but not the Ti. Your radio compares the input from both antennae and picks the strongest signal. For after-market radios just use the one from the back of the car with the more conventional-type plug and it should work fine. If your rear antenna lead has a short "snap" connector, you may need an adapter for a more normal old-style longer plug.
-Phil |