Having sussed the radiator problem (it had a hole in it) and fixed the speedo (just a broken cable, happily) I now have another opportunity for y'all to show how well informed you are. And you do it so well too

The heater in my '95 WRX works brilliantly. So does the aircon. The trouble is, the temperature sensor doesn't. The heater is either ON and the car resembles a Turkish bath in about 30 seconds flat or it is OFF and the aircon has kicked in instead. Which makes my fingers go blue.

Short of changing my clothing and/or leaving the windows open, anyone got any suggestions as to what I need to fix?

Ta muchly,

Simin