front speakers 01 impreza

Hello I just got my first car; a 2001 Subaru Impreza RX. The previous owner bought and installed an Alpine 9853R head unit, Alpine (Dont know which model)front speakers, sub, amp. Before they sold it they took the sub and amp out. The rear speakers have sound coming from them but the front ones do not. Its because the front speakers were powered from the amp that was previously in the car but is no longer there. My question is how do I wire the front speakers to be run off the head unit? I plan on getting a mono amp, sub, rear speakers and 4 channel amp in a few months. Thanks.

I had a look at the wiring then and the green and purple wires coming from the rear speakers are wired to the head units grey and white cables (green to white and purple to grey). There is some sort of harness/adapter which the green, purple, yellow, red, black, orange, navy blue wires from the head unit are wired into. The white and grey wires from the front speakers are also wired into this harness/adapter (Don't know what its called, sorry).

Anyone?