yea the sensor could probably handle the power to the motor for a short while, but will likely fail down the road. relay's are cheap anyways!
just get one rated to the amperage of the motor. motor amps should be stamped somewhere on it. get a simple 4 prong relay. the two high amp lines - one goes to ground and one goes to power. make sure not to switch them around from stock though otherwise you might reverse the flow like Vince said. smaller two wires - one ground & one goes to the sensor. if the sensor is a grounded sensor then you hook that up but give the relay 12v power at all times. hope that makes sense!