What the FW schematic should list is what other wires are bundled together through a given connector. That can help find where a wire goes. As in, a white/purple wire joins a bundle of others, whose colors are more readily visible on the outside of a wire bundle. That way if you can see some of the other colors you can narrow down where the one you need might be.
When trying to find wires it's VERY helpful to have a tone generator. I've got an older model of this one:
http://www.amazon.com/Extech-40180-Gene ... pd_cp_hi_3Or from monoprice, even cheaper:
http://www.monoprice.com/products/produ ... 1&format=2What these do is let you attach the tone generator to one end of the wire. And then use the probe to listen to where the signal goes. The probe works by merely being close to the wire with the tone being put on it. Not by touching the conductor inside the insulation. They're not cheap but they last forever and are an absolute God-send for debugging wiring issues like you've got.
You'd hook up one of the tone generator clips to the power wire at the amp (powering all boat electric off first, of course) Then poke around with the probe to hear where the tone comes in strongest along the bundles of wires. Eventually you can track down where it goes using the probe to find the tone signal.
These inductive tone generator/probe units are great for all kinds of wiring. AC, DC, coax, network, etc. They'll work on anything that's got a metal wire in it.