Stereo upgrade.. am I missing anything?

So I bought the base model with no back speakers, and I'm looking to add the missing back speakers, and a shallow woofer under the passenger seat. (amp under driver seat). The problem I have is getting the signal out of the OEM deck. There are no wires ran for the back speakers and I don't know how to splice into the harness to find the back speaker signal. I have had no luck finding a proper diagram for the three harnesses plugged into my deck.

So, my plan is to find a harness with the back speaker wires already ran, and splice the old one into it, which will earn me the two extra speaker signal wires I need.

THEN, run that signal into the amp
THEN, run that power to the woofer / two back speakers.

If all goes well, the OEM deck / steering wheel controls / fader will all function properly.

Before I go and try to tackle this however, I'm looking for some input or advice in case I missed something. As far as I can see, everything should match up if the harness plugs in . (getting a 1987-up harness)