Ok, Ive got an 05 crew f250 xlt. My cd player quit, and I found a great deal here on another factory just like mine. When I installed it, I noticed a second female plug on the rear of the untit. Whats this plug for, mine had nothing plugged into it, and Im curious why its there. Im wanting to add an amp/sub to the factory unit, and wonder if it can be done through this plug? Has anyone ever installed a powered sub under the rear seat of a crew cab? How do they sound, I dont need a lot, but a little more bass would sure be an improvement. Thanks for the help.