I was speaking to a stereo shop owner a while ago, and he told me something that is quite obvious, but I hadn't considered it...
When installing an amp you should run a power cable from the battery, then ground the amp to the chassis.
In some cases (Fords apparently) the negative wiring from the battery to ground is a bit marginal (He showed me a melted negative cable caused by a multi amp setup). So he advises that you should add an additional earthing strap from the battery to chassis (I used some 4 gauge)
Hope that helps somebody... =)

Jazz
(List 112, Mk2 12 gig #40. Mk1 4 gig #30. Mk3 1.6 16v)