I live in the uk so this might not apply to american cars, but we shall give it a go. On some headunits the wires at the back are labeled i.e Rear leftect. . If the channels are RCA format on both ends i.e amp and headunit just have a look at the back of the HU to find out which ones is left and right and the one thats left is the remote output. This of course doesn't help if the wires arn't coloured, unless you are going to follow each one to the amp to find out which is which. Which more hassle than its worth.