You've posted the answer to your problem and missed the bright red flashing light. According to what you wrote, it absolutely IS a grounding problem!

Set your meter to 2V scale. Keep the neg meter probe on the neg battery post and work the pos probe toward the alternator in this order...

neg battery clamp
engine block
alternator housing.

The most likely voltage drop will be between the block and alternator case. So the next places to measure (neg probe still at battery post) is each end of the alt. housing-to-block ground wire to find whether it is a bad connection between the wire end and bolthead, a poorly grounded manifold, or (most likely) a high resistance in the ground wire itself.

BTW - the abbreviations for polarity are + and -, or pos and neg, or plus and minus. Using "+ve" and "-ve" makes the reading confusing.