Thread: What are the rules of grounding?

1.
Ok I was grounding in spot in my trunk but it was a bad spot as far as permanence so I switched it to another spot. Now I get the odd noise when the cars revs up via the speakers. The only way not to hear this is to crank up the volume on the PC. Why I am I getting this noise? What are the rules of grounding something?

2.
I had my LCD monitor hooked up where the HU CD player was in the dash via the power and ground wires from the deck. Now, when I hooked it (the monitor) to my Laptop, no problems. But when I hooked it up to my PC in the backseat it had horizontal scrolling lines in it. So I took it apart again but changed the ground to attach onto the cigarette lighter ground, I got a 90% reduction in the lines on the screen when adapted to the PC. Why would the PC do this? (Yes I tried to ground the PC, alas, no difference. I tried several spots) Why would it make such a difference to ground the lighter instead of where the old CD deck was?