Steve,
To test you can take a direct feed and earth to the fan motors.
The motors can be stripped and cleaned or you can simply just purchase replacements.
Here are a couple of options for you.
Both in stock and good to go same/next day.
http://www.350z-uk.com/topic/66473-350z-mishimoto-performance-fan-shrouds-18000/
http://www.350z-uk.com/topic/46683-radiator-coolant-fan-motors-now-in-stock/
Just give me a shout if I can help you further.
Alex.