ok....for the questions........

common grounding means that all your devices share a single ground........even if you have 3 amps running just like in my van......

for ozcity's question.......

some amps have power delays.......notice that when you turn it on......it has a delay of about 2 seconds the most before you hear you amp running.....same thing goes when you turn it off.......as for the noise.........try to check your HU's ignition wiring.........same thing happened to me before.....I tapped my HU's ignition cable to an unreliable source.......(i never noticed that the wire I tapped into was also the wire that my A/C unit is using) that's why whenever I turn my A/C on......I could almost hear the blower in my system......try to secure a good ignition....the one with the less load........:D

for jake's question.....

double shielded signal cables are available in MG Infinity, Concorde and other audio equipment shops......just dont forget to measure your ride's length so that you would be able to buy the exact length of the cable.....:D