Another clue that is very likely related:
Rear fade & left balance: only rear left speaker plays.
Rear fade & right balance: only rear right speaker(s) play(s).
Front fade & right balance: only front right speaker plays.
Front fade & left balance: ALL SPEAKERS PLAY at full volume (although only the left channel of the track is played)!
Not only do they all play when they shouldn't, but they can go up to a much much higher volume without any amp clipping (some speaker distortion does begin). This is what I was expecting at no fade, no balance. So this is definitely the root of the problem.
This prompted me to check my wiring. The soldering I did from the head unit harness to the speaker wire adapter is correct. However now I'm not sure this adapter harness is correct in the pins that it connects to the vehicle harness. Here are my Ford harness colors (as currently connected through adapter harness):
Right rear - = grey / orange
Right rear += pink / yellow
Right front - = orange / black
Right front + = white / green
Left rear - = red / black
Left rear + = beige
Left front - = black / white
Left front + = orange / green
These do not match up with what I've found online
(or what was posted above).
I have found a wiring diagram
that seems to be more accurate to the colors I'm seeing. It looks like the negative/ground is shared between all speakers. Definitely not how mine is wired!