As long as everything is plugged in right, it is probably the head unit. I just replaced mine because none of the right hand speakers would play. Fixed the problem. You can pull the radio out and back pin with a voltmeter to check to see if anything is outputting at the speaker outs. If nothing is then it is definitely the head unit. I forget which wires are the right hand speakers. I think the speaker wires are the grey ones, the purple ones, the green ones, and white ones. Of those 4 colors there are two of each wire, but the second wire of the same color will have a black stripe to denote that it is the ground wire. I am not sure of the 4 colors which are the right hand speaker wires.
[This message has been edited by zkhennings (edited 07-11-2013).]