The taped off wires are for the "everyothermarketbuttheUSA" headlight dip switch relay.

To test the reverse lights, remove the black wire to the tranny from the relay and connect a jumper from that terminal on the relay to ground. Turn on the ignition. You should get reverse lights that then go off when you break the ground connection. Otherwise, yes, you need a new relay. If they work fine, the ground path through the tranny is disrupted somehow (wire abraded through, wire disconnected, little contact thingy in "doghouse" not making good contact, etc.).

However, as you said there was continuity to ground through the black wire, it would indeed indicate a bad relay.



--
Mike!