It is located in the rear on the driver's side behind the same removable side panel covering the Nav module, CD changer, DSP, etc. It is the silver rectangular box mounted to the DSP unit. Look at this video and you will see what it looks like.

YouTube - installing the BMW bluetooth module in a 2000 E39 M5

I first removed a small section of the flooring by removing three screws. This exposed the BT module. I next removed the mounting screws which allowed be to pull the BT module out. I then removed the two single wires connected to the end of the BT module that is visible when installed. I then removed the harness connector, waited a couple minutes, and reinstalled everything. Good luck.