I presume you have the CR or CDR 220? If so, here is the pin out from the manual on the back of the unit. Your black plug is in the right spot though as the rear speaker plug will only have 4 wires - 1 + and - for each channel as the doors and and dash, w/ the M490 or M680 package are driven by amp in trunk. Plus, IIRC, the rear speaker plug was brown.
If you have a different radio, you can download the correct manual from Becker using this link
http://www.beckerautosound.com/Porsche/Porsche%20Menu.html
Do you have a 6x40 amp? If so, they will be connected to it in the front trunk. Look at the bigger plug in the amp in the front trunk. If pins 5/15 and 6/16 are full, then they (rears) are plugged into the amp. So, maybe the connector plug is undone. Look for a brown plug about 6" above the gas pedal. Sorry to not have pictures to help, but I'm out of town and can only pull so much info for you.
Good luck.
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