Hello,
I have a similar problem, I have just spent the weekend doing busters mod...or as near as I could to my cheap chinese double din,dvd,gps, camera ( ebay 90 bucks!)
I went from no sound, to realising the amp wire wasn't connected to any power,(after sending 12v to it) to then as above ,volume..1 loud...2...well loud...3 defening....etc and a horrible hissing buzzing in the back ground. I have tried earthing the amp direct to chassis, and any thing on the back of the stereo that looks like it wanted earthing I connected up to a new ground screw straight in the chassis, but still got the problem.
Anyone got any ideas???? Its driving nuts...I'm new to stereo's by the way, so lets keep this simple!