Are you working with a known good harness, in stock form, or has it been butchered up in places? Having moved to a solid-state reg/rect, they'll generate more heat than the original discrete parts, and need to be mounted solidly to metal parts to help sink some of the heat. If you've only edge mounted it, you could be burning them up from heat. otheriwse, dirty of improper connections can cause failure.
You probably don't want to hear this, but check it all , wire by wire.. look for resistance where there should be continuity .
