2003 F350 Crew Cab Speaker Wires

I just bought a 2003 F350 and I'm trying to install my amplifier.  I don't require top-shelf sound quality, so the high-level inputs will do fine for  me.  I would like to avoid disassembling the rear speaker panels (involving seat belts, etc.).  I found the wire harness under the sill plate in the driver-side rear door, but I need to know which wires to tap to get signal.  The best info I have found is a 2002 diagram, which seems consistent with the available wires, but I understand that Henry is pretty inconsistent with wire colors.  Any help would be greatly appreciated...

It's big and its red, but it aint a bowtie anymore

Use a speaker harness, or take a look at this picture and match it up with the plugs in your car.  The white wire in the picture is the positive speaker wire, and the black wire is the negative speaker wire.

Or also you can use a multimeter to test for voltage across the two speaker wires while the system is playing.  Positive voltage means the wire touching the red test lead is the positive speaker wire, and negative voltage means the wire touching the black test lead is the positive speaker wire.

I used the 2002 specs as the wires in the loom seemed to match.  I don't have a DMM so I just hooked the suspected wires to a speaker... TOUCHDOWN!!!  Crude but effective.  There's no feeling like blindly tapping into the wiring system on a brand new $43k truck!!

Next question- where's a good accessory power supply?  On chevys you can just plug in to a blank spot on the board.  I found a WHITE/ green(?) near the brake control socket that had IGN power, but I can't find ACC power, much less retained accessory power.  Once again- any help is appreciated

 I worry some day you will learn the hard way that it's better to get a $15 multimeter from radio shack than blindly connect to wires and end up doing very bad things to your vehicle's electrical system.

lol anyways, when I go in to the shop tomorrow evening I'll check on my techsoft for the info.  Or maybe one of the other guys from the forum who has access to it will reply before then...