I've been unhappy for a number of years with the heater control in my 1992 4/4.
It had two positions: Hot and hotter.
Upon examination, it's a cylinder with a hole drilled from one side to the other, fitted in pipe. Like a ball valve, but cylindrical. It could be turned to either block or allow coolant to the heater. It was poorly made, and thus allowed hot water to get to the heater at all times. Fine for the winter, but not great in the summer, especially when hot and wet.
After some thought about using a central heating valve, I found a 12V motorised ball valve:
https://amzn.to/2YaPGMzI've just fitted it, wired up in parallel to the fan for the heater. I'll report back after testing.