I think theres multiple amp turn on wires incase you have the shaker 1000. one turn on for door subs and one for trunk subs(shaker 1000). I think the 12v - 5v keeps the subs from popping everytime you turn the radio on. I remember when I hooked mine up I had to put a resistor on the amp wire to keep mine from doing what im pretty sure that wire is that you have maybe?
05 TL Mustang Base (the silverskoundrel) - sold
05 TL Mustang GT - sold
98 UA Mustang GT - sold
10 CI Mustang GT - slow