Ok two whites are the heater, one will be a earth the other a 12v feed! If the black is the signal don't prope into it, the grey will be the earth for the sensor! If you are getting only 4 volts you must have a wiring fault! Do a voltage drop test, put muti meter reading volts onto the 4volt wire then to battery live it should read 8volts (4v+8v=12v) this would show you have a "voltage drop"