The two wires weren't hot at all, but they had +12V on them, according to the buzzer. The incandescent bulb has no polarity, so it worked, probably finding its own way to complete the circuit. I tested the wires with an LED and some test leads, and I think the hot wire was fine, but the one going to the rheostat is getting some voltage, even with the rheostat disconnected.
It's just confusing because I assumed all the gauge bulbs are in parallel, so why is the tachometer different?
I checked the wires to make sure the colors were the same on both sides of the connector, and they were- IIRC.
BTW- I'm using the same paddlewheel LEDs in all the gauges, but I just got some 24-emiiter paddlewheel LEDs , so I put those in the two tachos. There shouldn't be any difference in function; only the current and light output is different(slightly.)