The temp sensor is probably fucked!
Take the wingmirror off by unclipping the inside of the wingmirror from the inside of the car, then undo the screws and slide the wingmirror off. Aside from the wires that control the glass on the wingmirrow, there should be another two wires (+'ve and -'ve) that sort the temp sensor out. If you can find a cheap eleccy thermometer u can wire that up to see which bit is broken.
A temp sensor from Renault used to be about £80 new, prob best off going to a breakers, or I have one lying about I think.