I am in the process of trying to put in a new Pioneer Double Din Receiver I have all the cables currently hooked up except for the speakers and the reverse. The stock radio has the one cable that connects to the amp is there anyway to use that to connect to the new radio or do i just have to run everything to the amp in the back? Also, I know that the cable to detect that the car is in reverse is in the console but where exactly is it? Any help would be greatly appreciated.

 

did you not use a harness?

 

Is it set up yet? Get a harness from Crutchfield.com.... it's pretty cheap, and very easy to set up.

 

The aftermarket harness that runs from the head unit harness to the factory harness should have a wire that is labeled "amplifier" to run to your factory amp. Heres an awesome video that will hopefully help you out a little more.
Car Stereo Wiring Explained In Detail:

 

The reverse cable is the white and green stripe running under the door sill.