I have a g35 with the bose system but with a pioneer avic-d3 HU.
This has not happened before and arised a couple months ago.

if the HU is turned on at low volume, the left speaker will not work but will cut in when the volume is set to 9 or above.

and after that if you turn it down again, it will still work until very low volumes like below 2 or when you turn it off.

It happens almost everytime and I know it happens in G35s with the bose HU but the problem is I have an aftermarket HU. this shouldn't be happening.

What i would like to know is that if this is a HU problem or speaker problem?

 

Chances are it's the Bose amp, not the HU *or* the speakers. Failure of the Bose amp(s) seems to be a fairly common problem. If you'd really like to troubleshoot it though, the first step would be to switch the left/right channels at the wiring harness on the back of the headunit. If the problem switches sides, you know it's in the HU. If it remains, it's in the Bose Amp or speakers.

Good Luck.