Lexus RX 2010-15 :: Sound Comes Out From Front Speakers Only

My mother owns 2011 Rx350. She bought it used few month ago. Car's great for what it is and makes me feel relieved to think my mother's driving a car that is quite safe and reliable.

That being said, the car's audio have created some problem recently. How it happened other than my mother's assumption that her water spill at the trunk may have caused the problem. She don't know how much water was spilled and not even sure if that is exact reason. (It's about that time frame that audio started having problem, hence the assumption)

So, the problem is that the sound only comes out from the front speakers. I am not sure how many speaker there are at the front, but it is as if fader is set to front all the way. If I move the fader to rear, it kills the sound from the front and since there is no sound at the rear, it just dies out as soon as fader moves all the way to the rear. So with the fader and balance at the center, sound is only at the front and plus there is no bass sound also, assuming subwoofer (if any) is at the back and not working also.

My mother claims that it comes back to normal sometimes but I am not sure how often that is since I do not drive that car too often and I just found out the problem today.