Headunits normally have their own propitiatory connector on the back. They come with a loom that goes from that do an industry standard ISO connector.
Some units have these iso connector on the back, if your lucky that may be the case, otherwise you'll need to source a new loom.
ISO connectors look like this:
The black one (left) is for power, remote on, dimming, earth etc and the brown one (right) is for speaker connections. Very often these are all in one plug like the left plug on this:
EDIT: You can get an iso connector in halford if it fits then just chop it up
