Before installing the motor you have to solder the wires. Now, how to solder the wires based on polarity??..... I have an idea. These motors already have a set of screws which connects the yellow casing. When you look at those screwa in such a way that the motor is on your left side and the shaft is on your right, then most probably the top terminal is GND and the bottom is +. This is from my experience.
