Originally Posted by tabetha

Try disconnecting the overide switch and see what happens as it takes so little current to keep a coil winding(RELAY) energised, that the switch could easily be doing it, ie leaking enough current to keep contacts closed and fan on.
I would also first try another relay if the contacts(inside) are too close or the return spring is weak inside, the coil winding contacts have a NATURAL tendency to remain closed, called unintentional latching, very common on some relays.
tabetha