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After replacing my GPCM tonight, I decided to take the old one apart and see if I can figure out why it stopped working on the passenger's side. Popping the bottom cover off reveals a circuit board, encapsulated in some gooey stuff (the technical term), like this:
Note the bundle of five traces close together on each side. A close up of the green (drivers, and working, in my case) side looks like this:
On the black (passenger's, and broken in my case) side, the traces appear to have fused together:
I suppose these can fail in other ways, too, but I bet this is common.