Looking for help with identifying radiator fan issue on 95 Vectra B hatchback 2.0l

Hi.

A friend of mine owns an old Vectra B. Recently the radiator broke. He had replaced it, but now the fan doesn't start. Sometimes (rarely) it starts but doesn't go off afterwards. He thought it was due to the temperature sensor at first. Replaced the 4th time, but they seem to not work. Now, after so many tries he thinks that something else may be damaging the sensors.

Any recommendations for what to check / fix next?

Thanks for your help in advance.