Just for fun, I peeled back some of the tape. Found one of the blue wires for the switch plug tapped into the blue wire for the seatbelt connector. The seat belt light does work properly in the car right now, so apparently the blue pinout for the seatbelt/mystery switch is there in the right spot. The other switch blue wire goes into the conduit.
This makes me suspect that this pinout is the missing one on the new harness (red circle above). Tomorrow I'm going to remove the switch wiring/plug, pop off the plug cover on the new harness and try to install the pinout where it would have gone on the old plug.
If it works, I'm probably just going to tape up the switch somewhere out of sight since I'll never have to use it, but I might take out the seat and install it right. Not sure yet. We'll see how tough it is to do.
If anyone has some better ideas, I'm willing to listen.