Here's an idea.
https://www.youtube.com/watch?v=NVysqXm81aw
Years ago I installed an electric fan that came with a thermostatic sensor switch that could easily be attached to the radiator and also controlled the temp. at which the fan came on. There was also a manual switch under the dash to override the sensor.
Any reason you can't plug your fitting at the bottom (wrong location) and just install a sensor that mounts on the radiator and can be controlled with a manual switch if desired?