That's why 131s should be RHD! The heater valve controls always break with the heater stuck on in the summer or off in the winter! But in a RHD car you can simply reach under the dash and turn the heater valve on or off manually!
The problem with the cable controls is the the small plastic clip (on Series 2 and presumably Series 3 cars; Series 1 cars had metal clips that lasted much longer) that holds the outer cable in place breaks off. They were available as spares years ago (I bet Fiat dealers made lots of money selling those) but I doubt you'll find any now. The best way to repair it is a large cable-tie to clamp the outer control cable in place and some Araldite to stop the cable from sliding.