The links below are for the combined WIS and Wiring Diagram to do a Voice Control Module retrofit on a 2001 W163 ML and the retrofit publication on benzworld.org.  The publication was written for my 2001 ML-320 but should  be the same for all 2000-2001 W163 models.  I have also included links to two other webpages that proved very useful for my retrofit.  None of this information is required if you have a 2002 or later ML.  The information on how to do a retrofit on those model years is included on the Phone Install Bulletins you can download from the Phone Bulletins page (under "Integrated Phones").