IIRC, Kary (BigBlue) said that he had taken a harness apart and the two wires are tied together within the harness. It doesn't make sense to me for MMC to make individual harnesses just to provide for the difference in models, so you might find the wire taped back inside the harness loom at the first junction back toward the firewall. If it's there, the next check would be for continuity to the gauge wire. If there's continuity, then the two wires are indeed connected within the harness. If not, the other end of the idiot light wire should be taped back inside the harness at the first loom junction of the gauge wire.

It would be great to be able to tee off of the filter adapter and use both senders with gauge and idiot light. The idiot light could also be wired in parallel with a loud buzzer alarm. I always wired boats that way and it was so much better than either system alone.

Frank