temp sending unit ground problem

ok, temp gauge hasnt worked since i purchased the boat and went to replace the sensor since the gauge tested to be ok, while I had the test lite out i figured i would just test the path to ground, I have no Ground at the thermostat housing, which the sensor screws into.. The intake is a good ground, and the cover plate on top along with the bolts are both showing as good grounds, the housing wont complete a ground circuit. their is a gasket between the intake and the housing, and a gasket between the top cover and the housing, so it basically is insulating itself, how should i go by fixing this so my gauge will work? Thanks...