The defroster cable for a hardtop or glass window top will be on the drivers side in the US, the rear speaker cable would be on the passenger side. The rear speaker plug is there to allow you to remove the rear speakers/storage shelf when accessing the engine.
The cable you have pictured looks like the right one, one end is for the rear speaker plug, then it is routed under the passenger side sill and across the passenger footwell to the Becker head unit. The plug you are holding in the picture is the plug for the back of the head unit. If you have a 6x40W amp from a 996 you can also use an adapter to connect the rear speakers directly to the amp. The only difference between the two is the amount of power provided, I think the HU does about 50W watts.
The CD changer doesn't have anything to do with the rear speakers. They are pretty useless these days, unless you want to use it wire an AUX input if your Becker HU doesn't support AUX.
I do think it is very odd you have rear speakers and no cable, I would search the area around the passenger side of the engine bay and behind the B-pillar trim that covers the seatbelt (you will have to remove that piece anyway when you install the new cable).
Search rear speaker install instructions for details on installing the cable. If you want a diagram/pin out of the back of the Becker HU you can also find that by searching the HU model number. If you have an aftermarket HU you may need and adapter, unless you want to use the pin out to wire it yourself.
