The reed switch sensor just clips into the bottom of the expansion tank. It never gets wet as there is a magnet in the float inside the tank. It's easy to replace, just pull it out from the base.
It is a valuable modification to make to your 940 as it has saved my engine a few times when a coolant hose/heater control valve failed. The light coming is a scary thing.
There is an guide in the articles section on the mod that I used to fit the sensor on my car:
Linky
The rectangular expansion tank should already have the float inside. I sourced the reed switch and relevant loom wires from another forum member that was scrapping an S90 (V90 also had them).