You should order the Metra 70-5605 amplifier bypass harness, the black plug will connect to the large plug from the amp (that is the speaker harness.) Then you connect those wires to the head unit's Speaker wires. The Gray plug provided with the 70-5605 gives you a connection to the "large plug" you mention, for switched power, antenna, dimmer. Attach that to the appropriate wires on the head unit's Power wires. The yellow wire from the head unit should be connected to the battery or maybe the power seat circuit, because the Ford radio wiring is for a low power radio.
