It depends how the fan is wired. If there's no relay then there may be a thermo switch in the radiator header tank. Late MGB's were activated this way, so there are two wires - live in to the thermo switch, and switch to fan from it. A wire from the switch to fan wire to a light, then to earth would work on this system.
Where a relay is fitted it's usually activated by the earth wire through a thermo switch, (Otter type)so a single wire is used. All this needs is an ignition switched live feed to a lamp with a second wire connected to the single earth wire to the thermo switch. Use a multimeter to find out how it's wired first, and be careful. It's very easy to blow fuses without much effort.
