Why wouldn't you consider fitting in a 5-channel amplifier and use the fifth channel for the sub ? Therefore, you would need only one power cable coming from the engine. This is how I have made my install.
The head unit does not have pre outs, so you need high-low converters, from the audio cables you should connect pre-amplifiers which will output rca cables for the amplifier.
I used these by Phonocar :
http://www.phonocar.it/easysite/sche...lingua=inglese
Few days ago though I installed a different head unit and removed them. But they worked great with the factory head unit.