How does one replace the reaar light harness? Do I splice it to the intermediate harness? Or is there a plug hidden in the intermediate harness plaastic cover????

There is a connector (plug) in the trunk. From the left tail light follow the harness forward, you'll find the connector just behind the rear seat.

Just did mine tonight. Where the rubber gromet is on driver side, there should be the conector maybe 10 inches up the wire from that.