AMP is short for AMPLIFIER. You may have already guessed that it is for an amplifier. The AMP wire is often blue with a white stripe along its length, but check the label to be sure and use it to switch the amplifier on when the headunit boots. Your amplifier instructions will explain where to connect the amp wire to. Don't draw very much current from AMP, it is to switch the AMP on and not to power it.
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