CD Changer interface
 

I am trying to install an alpine s620 CD changer in a 91 420sel. I have found all the stuff I need to do it but my question is: What does the "interface unit" MB part #BQ6 82 0135 consist of ? Is it just a cross connected cable or are there electronic components of the type that might be needed to convert serial protocol between the head unit and the CD changer. If it is simply a cable I plan to build my own since MB wants about $150 for two cables and another $150 for the "interface unit" But Im not sure if the interface unit is a passive or active device. Has anybody used one and what did it look like ?? Is is simply a cable or is there some sort of electronics involved ??

Thanks.

It is a controller of sorts (black box with power and ground and 2 DIN cables going to it). I'd go ahead and buy it or replace the head unit with a better one.