Try to identify the + (hot) Positive wire going to the meter, (speedometer). Find the - ground.
Also, download the manual here to get a look at the schematic.
Follow the wires back from the meter. Somewhere, there will be a closed connection, making the circuit work.
I'm not sure of this, but, RE might have the hot on all the time. That means if the ground wire grounds out, the circuit goes on.