MOST Fibre optic interface to allow the fitting of a new head unit into a  Mercedes ML, GL & R series and retain the use of the fibre optic amplifier.
You will lose front to rear fading ability. Also works on some Porsche cars

On Porsche
Only for vehicles with the CDR23 CDR24  PCM2 or PCM2.1 original BOSE audio systems. NOT for PCM3 systems.

if the car has speaker wiring in the ISO connector or Quadlock then it is NOT a fibre-optic amplified system and this interface is not needed

This interface does not retain the use of the steering wheel controls.You should use separate steering control interfaces 49-POR-01, 49-POR-02 for Porsche cars  and 39-MB-UNI for Mercedes cars.