actually no, the ballast resistor idea won't work based on feets explination.
To send a pulsed signal you would have to wire in one of the dash voltage regulatros. But that is a 3 pronged item.
It would send a 12 volt on off signal.
I would bet the relay is still built into the motor if there are 4 wires coming out of the motor to the box. So you are still good with your old wiring.