Yep it would be the head unit if it is factory. I would replace the speakers, as their sound is very sub par, but you dont have to. The fronts are easy to replace, the rears not so much.
You can pop off the front speaker grills and unscrew the speakers, unplug them and swap them to see if they work.
[This message has been edited by 2.5 (edited 09-25-2013).]