I will absolutely check all the grounding combs - great suggestion! Now that you say it, this does have the stench of a bad ground somewhere. And yes, the alternator was replaced (by me) roughly a year ago. The ground has a plastic nut and I'm always a little nervous when I tighten it. Maybe it's working its way loose? I can't think of anything else I've done that could affect the other grounds. But I'll check all the ones I can find/reach.

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2002 X5 4.6is - Imola Red/Black (Sold)
2001 740i M Sport - Anthrazit Metallic/Black (Sold)
1998 M3/4/5 - Technoviolet/Dove (Sold)
2003 Z4 3.0i Sport - Sterlinggrau Metallic/Black (Sold)
2009 Mini JCW (The Wife's)
2015 i3 REx - Arravani Gray/Tera World (Daily Driver)