I take it this is an LS430, not an LS400 (I've looked through Toyota's Electronic Parts Catalog and the LS400s appear to have a single fan that's driven by the engine).
The fans are controlled by the "cooling fan computer" mounted nearby. Look for a small, finned block that's mounted at an angle near the radiator. Part number is 89257-30040, fits LS430 from August 2000 onwards, and nothing else. Worth checking its connections.
Also there's a temperature sensor down to the left of the throttle body as you're looking from the front. Apparently it's a water temperature sensor for the E.F.I., but other stuff might read it, too. Parn number is 89422-30030, once again it only fits the LS430.
Hope this helps.