I have a 07 Mustang GT with the shaker 500. I will be using the metra harness and all the great info I have found here. I do have a sub/amp in the trunk wired into the system. How can I hook this up to the metra harness as well. Will I need to get an rca splitter from the harness to run off the avics sub out or is there a better way? Thanks

metra harness 70-5521 has the plug you need. you will also need a 3k ohm resistor on the blue/white trigger wire to get rid of the "thump" when powering up

Thank you. What about hooking up the aftermarket sub/amp combo I have?

i just told you that