I have a 2003 g35 sedan, I have had this problem for a while now but it has gotten worst lately. The radio works fine every time I leave the car overnight, and the sound works for a few hours, but then when I try to lower or raise the volume it stops, or the volume number in the radio changes but it doesn't affect the speaker actual sound. Once the sound stops I try to raise the volume all the way up or all the way down and all I hear are random speaker pops or only sound from one side of the car. Also sometimes the volume is stuck at a specific level, and even though the radio shows the volume number changing, the actual sound stays the same. So I am assuming the speakers are not blown out because otherwise, they wouldn't sound at all. I replaced the amp with another stock bose amp that I got from a junkyard but still have the same issue. Any recommendations?

 

Pretty sure the head unit is the problem.

 

Bench test it!
Any good quality automotive audio shop would be a better solution for you rather than guessing and just throwing parts, wasting time and money.

Good luck