^^^^ sorry, i need to explain more...i just re-read it and it was confusing to me too. :P

inside 90% of the amps out there, the signal ground (the RCA ground) and the amps power ground are not isolated. if an amp is incorrectly grounded (or not grounded at all), its still possible for the amp to power up and work (although not to full potential) because it uses the RCA ground as its main ground wire. unfortunatly, the RCA ground is also not isolated in many headunits, so the ground signal travels through the RCAs, beck through the headunit, and "out" of the headunit ground. naturally, the headunit is not designed to carry this much amperage, which can lead to all kinds of chaos and usually results in destroying the headunit output stage.

somthing EVERYONE installing an amp should do is this: before plugging the RCA inputs in, just connect the +, -, and remote connections to see if the amp turns on. if it does, then you can be sure you have at least a decent ground. if it does not, you need to find a new grounding point (assuming the + and remote wires are OK). many people end up grounding through the RCAs and think everything is fine till they turn on the stereo one day to find out it only hits 45 dbs. by first isolating the "power" section of the amp and making sure it functions, you can alleviate yourself of trouble down the road.