Noise typically results when one ground is backfeeding into another due to their measured differences. If you have two grounds that are equal then there will be no backfeed. However, trying to accomplish that in a vehicle is near impossible. Typically multiple grounding points already exist in a vehicle. You could use separate grounding points to maintain the "18 inch rule" but then that idea of cross talk between differences in grounding points could occur.
Same thing can happen with a single ground point. Car Audio amounts to nothing more than good guess workPick your poison. My grounds hit a block that feeds to 4awg grounding point on rear wheel well. It's about 18" in length. The biggest reason for that rule is so that your cable can carry a large load without frying.
Remember...the difference between grounding points could even cause back feed through RCA's. That's why EQ's traditionally add noise to signals. This is why people prefer digital as much as possible too. I believe balanced and digital are over kill in a vehicle environment.
