wire/lamp identity??

There was also a separate bulb socket for seatbelt warning added. I don't know exact year it happened. 73 has it. I think the metal inst' housing was used to ground it so it needed only 1 wire, otherwise there would be 2 wires.
I don't recall blue with white tracer being fused beyond the fusible link. If that's the case here a lamp wire can stop the whole show. Ignition and charging system are also on blue w/white.