There's nothing there.
The kits include all the wiring to run from the rear speakers to the front.
You have two choices at that point ...
1. Easy but not as good is plug wiring into back of radio (no amp connection)
2. Better is to obtain a 6 channel amp as used in 996/911 and connect wiring to this amp's rear speaker terminals. Recommend amp from 996 Cabrio as opposed to one from 996 coupe.
The 911 amp is called 6X40 and the amp you have is called 4X40. The four channels are front left/right and subwoofer left/right (door speakers are subwoofers). The 4X40 amp is not designed for rear speakers. However, rear speaker outputs from radio to amp exist in the wiring harness, but outputs from amp don't. For that, you require the 6X40 amp.
See
http://www.renntech.org/forums/topic/3389-pinout-for-6x40-m490-amp/