I recently purchased a B8200 that already had a gauge installed. Mine happens to be the type with a capillary tube from the sensor to the gauge rather than using electricity but style shouldn't matter. The sensor is installed in the side of the water flange by the thermostat. The parts diagram shows a threaded plug in the hole where it is installed. There is a short pipe nipple inserted where the plug was removed and the temperature sensor was screwed into the end of the nipple which may have been necessary due to the length of the temperature sensor.
Beemer