RED-CLK

I'm planning to install a Ebay Comand unit in my 99Clk, and after a bunch of research I'm just as confused as ever, as to what extra wiring if any, is going to be required. The GPS antenna won't be a problem as I can use the Alpine dash unit.After I removed my radio, to see just what wiring allready exists back there, I noticed that there is an extra four connector flat plug-in that is not used. Apparently CLKs are pre-wired for many options as I have recently installed a CD changer in the trunk and the optics wiring harness was already in place making it a plug and play installation. Can anyone tell me what that plug-in is used for?