I'm not sure if what I'm going to add to this thread will helper hinder things but with my mechanic's and auto electrician's hat on, here goes....
Renault engines were fitted with a sensor which controlled a warning lamp on the car's dashboard that indicated the engine was too hot. The was set (from memory) at 110 deg. C, but I might be wrong about the number. Either way, if the red light came on it was usually too late and your engine was already too hot. The red light came on by the sensor shorting to earth at the pre-set temperature, thus igniting the bulb. The cooling fan was controlled by a separate sensor, which was set at a lower level than the warning lamp, so (in theory) the fan's actions would preclude the warning lamp from lighting in normal circumstances. The fan would come on & go off as the engine heated & cooled, but the light should not come on in normal driving. Fitting a temperature gauge allows one to monitor the temperature and see the engine temperature rising before it becomes a problem. I'd fit the sensor for a gauge in the same place as the warning light sensor and do without the warning light, or perhaps use an adaptor to allow 2 sensors to fit. Either way you don't need to drill another opening into the pump. The gauge's accuracy can be verified by wiring it via a 12 volt supply & placing the sensor in a saucepan of water. Heating the water to boiling will indicate 100 degrees C., so see what it reads as the water is boiling. The car's wiring must be in good order to ensure accurate readings, so check the voltage to the gauge is 12 volts all the time & check the earth return is sound, too.
Sorry to have gone on a bit.......