Up for sale is a fully functional, good used BMW CCM control unit. The function of this unit is many. There are about 30 separate, but identical test circuits in the CCM that check everything from your brake lights to the key in the ignition. Every single one of these test circuits is set up as what is known in the business as a "current loop". That means that the system sends a current through the closed loop of the and measures the voltage drop across a precision resistor. If the drop is out of range or the current does not pass through this unit properly (say you have a bad saulder joint), you get an error or a potential malfunction. This primarily happens with the cars lighting systems but can extend to others as well. These CCM's fit a variety of BMW cars, best bet is to check the part numbers off of your original and compare it to this one. 

5DS 005 138 00 (5DS00513800) / 61.35-1379379 (61351379379). 

 These are very expensive to buy new, and are excellent to have for diagnostic purposes, or if you have faults you are trying to correct. Thanks for looking!