hi craig,
ive seen the level sensor on XM rads before, it is indeed installed in a bung on the top tank. it is possible that the radiator was replaced at some point with a "universal-fit" or simply incorrect fit radiator without this provision, an dthe ring terminal was just left to hang loose and is shorting out on the frame. you will find the wire for the level sensor poking out of the wire harness that passes over the top of the radiator.