SOLUTION: The circuits were crossed, but not in the
dash harness where I thought.... tail light sockets

The connections in the tail light sockets were so
loose that they must have been touching each other.

The plastic block that holds the tail light & running light wires seperate has wide
slots cut into it from 1 side rather than just countersunk
holes for the mushroom connectors to sit in. I think
this allowed too much play in the wires, and since the
last 1/4" inch is bare metal some crossing was going on.
I wrapped the crimped part of the wires right up
to the mushroom and no more shorting.

All in all, i'm glad the new harnesses are wired correctly but