Sure, some of the symptoms came and went, but my parking lights on one side turned on randomly when the car was sitting in the garage, turn signals stopped working, headlights were permanently on and the switch wasn't working, no gauge lighting, and a few other things. I thought maybe it was a low battery, and the symptoms went away for a bit when I changed it, but came back pretty quickly.

@Clavurion I think you're on the right track. I tried changing to .C18 and .C20 to see if those worked and got a different error message (with .C20). I grabbed another LCM at the junkyard last week with the same PN as the original one, and it coded right up on the first try at everything, so I'm sorted for now. When I have more time I'll plug the other one back in and see if I can't figure it out. Thanks for your help!