Recently upgraded my shaker 500 system to a Pioneer X4500BT on my 05 GT. Bought from Crutchfield which included the main harness, a subwoofer harness and a 5volt converter. Bought a video bypass circuit from another source as well. Install background limited, ( Lots of internet time ). Installed with no problems every thing sounds great to a point. Noticed that the door subs rattle on the mid to top side of the volume. In other words can't crank it all the way. Noticed that on my old Shaker HU that there are 3 ports for plug ins. This means I am left with an open male plug hanging from my mustang. My local radio shack told me Crutchfield sent me a cheap adapter since it was FREE. They showed me an adapter that had 3 plug ins, however, they were not very clear on the reasons for the third one in question. Also their adapter was $ 140. Any help appreciated. Thanks

 

refresh my memory- are the connectors gray or black?

 

All three are grey

 

OK. The largest connector is the main one, it has power, ground, speaker outputs, and switched accesory 12 volts. The small 8-pin connector has the audio signal and enable/clip-detect wires which go to the subwoofer amps. The third connector is for external accessories and communication; it has things like wires for the aux input jack, Sirius radio module (if the car has one,) the CANBus wires, etc. I don't think there's anything on that third connector you need to worry about with an aftermarket head unit.