I can't tell you where the passage is, but I can alert you to one hazard: If the chassis ground connection becomes resistive, you are likely to blow up that nice new amplifier.

Here is a way around it - simply put a fuse in the 'ground' or negative lead as well as the hot lead. If the factory engine-to-chassis connection becomes resistive, the ground cable fuse will keep your amp from becoming the defacto ground return for the starter.

Just a suggestion, I know that fusing the 'ground' or 'return' line seems silly - but it has saved a few pieces of aftermarket electronics over the years. ANYTIME you run a ground wire direct to the battery, always fuse it. What ever rating the amp calls for for it's power lead, use the same rating for the negative lead as well.