Like Tony said, constant power (usually yellow), switched power(usually red for the car harness, but orange on the empeg) and ground (black) to both units. You may also want to hook up the headlight sense to both units. Then the amp remote wire to the empeg.

Then connect the pre-amp outputs on the back of the existing head unit to the aux-in on the empeg. Then run the front and rear outputs from the empeg to the amp.