Trying to expand my harmonic palette here, and came across this song.
Chorus of Stevie Wonder's "Don't You Worry 'Bout a Thing" (in E♭ minor, but temporary shift to relative major, G♭):
G♭ G♭7 B♭m7 C♭maj7 A♭add2 D♭9sus G♭maj7 B♭(ALT)
I'm pretty sure I understand the rest of the song's harmony: stepping into the relative major, then we end up on the IV chord, then II-V-I, and V-i back to the relative minor for the verse. The way we got to the IV chord, though, is a mystery to me. I'm confused about the particular sequence [G♭7, B♭m, C♭], however. I can see the I7 (G♭7) chord moving to the IV chord (C♭), but the iii chord in-between is stumping me.
I'm picking up that the iii-IV move sounds nice, so from that angle I can see why the iii goes to IV, but the I7 going to the iii sounds kind of strange.
I'm currently thinking of the chords as a I7-IV movement with a passing chord in-between, but that seems strange to me, because the harmonic rhythm has this chord land on a strong part of the phrase.
I guess my other possibility would be a deceptive resolution to the iii chord, followed by some sort of delayed resolution to the IV, where I feel the song is pulling towards...
What would you guys do to analyse this progression?
(For starters, have I misheard the chords? Is there a better justification for the chords?)
(And please, let's stay away from "if it sounds good, it is good". You and I both know that; if I were looking for that kind of answer, I wouldn't need this site to tell me about it.)