Thanks, that's very helpful.

Although, it just occurred to me that I remember seeing a fuse in the glove box fuse box that was associated with the navigation unit, so before I go any further, I think I'll just pull that fuse (and any others that relate to the stereo/nav), keep the charger on overnight, and see if the battery doesn't charge up. If it does, I'll know the parasitic leak is confined to the stereo/nav circuit, probably the unit itself.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT