behind the grundig head unit is an adaptor piggeybacked(3 block iso),so remove this that leaves you with standard iso plugs(1 for speakers and one for power)then the blue lead that goes to the boot cd unit is compatable with pioneer cd changers(older type,not sure about new range)
i have a pioneer KEH+m9100 head unit and CDM25 6 disc cd changer
works great with the original speakers fantastic sound.

but technology has moved on !!!!!
you can now get a modulator that you plug in,tune your radio to it's station
plug in your iPOD (or most mp3s) and it plays through your car sterio.
so joining cd changers together is not worth the cost or hassle (you need a 20 way relay and splice the looms)
you can also plug in mobile phone mp3 players and usb mp3 players to a modulator if you get the right one.
search "modulator" on ebay
