I used the Metra Axxess ASWC for my 05 GT w/monsoon and it works awesome and auto-programs (although you can also manual program).

You pretty much connect 4 wires and BAM!

1. Black to a ground.
2. Red to 12v accessory.
3. Orange/Green to FACTORY BLUE wire - (A6 on FACTORY harness) - have to splice into this.

4. Last step is to run a jumper wire from your 12v accessory to the PURPLE wire located right next to the BLUE Factory wire as stated above ^^