if it uses a fan switch, then just permenantly earth the switch feed to trip the fan relay constantly.

Only do it if your fan relay is powered by an ignition run only circuit.

You dont want it running when ignition is off and you dont want it running during the start phase.

In fact i cant see why youd want it running constantly in the first place..... it wont be doing anything whatsoever except loading up your alternator and draining a few bhp off the engine until the thermostat opens.