Before commiting yourself as to where to place the earth use a
multimeter and ensure the resistence is 0 ohms. Also how can you
be sure it is comming from the earth and not the RCA's or Stereo?
Don't mean to sound like a wet blanket but the interferance could be a
number of things, I know what a pain it can be, as it happened to me
last month when setting up my installation. Securing the
earth to any point in the boot as long as it is solid and unpainted
should be fine, idealy the best way is to go from battery negative to
bolt to bolt 18 inches apart until the amp earth point but then again
nobody does this me included. Good luck mate and hope you get it
resolved quickly.