How do I change my decks output from FRONT to REAR?

[Mr. X]

I have an aftermarket JVC deck installed on my car. The volume is too low. For example I turn it up all the way to 50 (the max) and you can barley hear it. When a relative of mine put his unit in my dash (He took out mine) and his sounded loud coming out of my speakers. He said its because his deck has a front and rear output and mines only has a front output. So I wanted to know how to make my deck display REAR instead of FRONT, maybe that would solve my volume issues. ANY HELP WOULD BE VERY APPRECIATED!!!!!!!!!!!!