Originally Posted by macguyver

A resistance test is absolutely meaningless. A multimeter sends out millivolts and milliamps as test current. Poor connections, or connections that will not actually pass large amounts of current will not be found this way. I promise you that a joint that reads 0.0 ohms with a multimeter will read something very different at 50, 75 or 100 amps.

DC is not like AC at all. AC will often bridge a poor connection, but DC will not. Even at lower currents, DC will cause a poor joint to heat up, and increse it's ESR quite substantially.

The only 100% guaranteed good ground you will ever have is to have both positive and negative lines run all the way back to their source, both battery and alternator. Anything less and you're banking on a the quality of another's job that really we have no idea how good it is, until there is a problem to fix.

Current flows down the path of least resistance. If you have 14.4V potential to ground at your deck, but you are getting 13.5V at your amp due to current draw, porr gorund or inability of your chosen ground path to pass current, you are going to get circulation of current and voltage along the RCA lines. As RCAs are passing AC, this is where your AC ripple from your alternator is likely to pass. And that 0.9v difference may be more than the signal your deck is producing. The effect of this would be noise in the extreme, but most guys don't have that problem, their problem is more in the microvolt range, but still enough to be annoying. This is why grounding the RCAs can be effective, but not always, to remove any stray voltage on the RCAs between components. This is a "ground loop" and the reason "ground loop isolators" work is to break this physical link with a 1:1 step transformer.