Problem: Voltmeter in dash goes left at idle when defrost and blower on high (headlights on, but they're always on). Usually voltmeter hangs about 1pm)

Did some multimeter readings in engine compartment:

Battery post to post reads 13.0 with above on. 13.7 with only headlights.
Battery reads 12.7 when car off.

After reading FAQs, I think my concern is with these readings:

Alternator case to battery (-) is .33v
Battery (-) to engine block is .03v
Alternator case (-) to engine block (-) is .31v
(all above with headlights on, car idling)

I'm thinking this points to a bad ground cable from battery to engine block, the main ground wire??


(Side note while doing this testing, the exhaust at some point started getting very rich, stinky, almost blue; turned on car 15 minutes later to double-check multimeter readings and exhaust was back to "normal")