If you absolutely cannot find the main-unit, the only thing I can suggest is looking around on the Panasonic website.
Because this is a propitiatory cable to connect 2 pieces of equipment together, it is highly doubtful that someone will have/make an iPod or any other type of patch cable for that plug. Especially considering that main-unit has enough audio-inputs for a couple video game systems and an iPod...
If you're at a dead-end after you look at panasonic like I think you will be, there is one more option.
This will require lots of time and patience, and if you get cranky at night, it may be best not to post anything... considering you may need considerable help with this potential last-resort solution, and pissing someone off will leave you dead in the water.
To sum up what I'm talking about, it would require taking the units apart to map out what pins are possibly connected to what components.
There are 25 pins in that plug, and only 6 speakers... so if it were merely an audio cable, half of the pins would be unused...
If this cable carries anything other than analogue audio, you're SOL unless you wan't to sink even more time and money into it to reverse engineer the communication protocol. To make things a little more complex, even if it's analogue audio in that cable, the main-unit will most likely be responsible for all audio processing. So any simple patch cord will merely run whichever 2 speakers the stereo out on an iPod are connected to. The only solution to this is building an audio processing device to operate a front and rear pair of speakers, as well as a centre and sub. It's not simply connecting them all together, as different frequencies will go to different speakers. Building an audio processing device is well beyond my abilities. (Although a shortcut could be made to just connect the speakers together... but quality would be horribly reduced. You would also need to rely on using an Equilizer on the iPod to make it sound half decent... as like I said, audio processing is most likely done in the main-unit)