Gooch, it's the "fatter-than-normal" looking ABS unit on the left fenderwell, with about 5-6 stailess brake lines running out of it, about 4 on top and 2 slightly beneath them.
Does the ASC light go out by itself after the car warms up? Have you tried cycling the ASC with the dash switch?
The BMW TIS shows a number of potential problems including poor electrical contact to the wiring harness for the ASC controller (in the big, E-box with the DME) to problems with the ASC throttle valve actuator. Bill or Winfred may be able to provide more info as the TIS says to use the DIS tester...
Cheers,
Duey