In my 93 Accord SE, I've been having recent trouble with hearing alternator noise through my left door speaker. After much research on the internet, many solutions say that this is because the stereo is not grounded properly, or the grounding point has become faulty, etc.

The question is, where exactly do I find these grounding points? When pulling out my aftermarket stereo, i just see harnesses attached to other harnesses attached to other cords, I have NO idea how to find out where the grounding wire is actually connected to, whether it be the chassis of the car or the negative battery terminal. Since many online solutions say to just "simply change the grounding point" or "clean the grounding contact point", I need to find out where the grounding points actually are.

Any ideas?