If they haven't been "rigged" to run all the time, then your thermostatic switch could be at fault. I couldn't be annoyed with the fans running ALL the time, so I just connected them to a toggle switch in the glove box which allows me to decide when they come on. If I forget, the thermostatic switch will turn them on automatically, but for track days I switch them on before heading out on track.
Cheers,
Dave