Is the service to the fixture 110v or low voltage? Is it under a porch or out in the 'weather'? What's it mounted to/on?
To test the wire, you're best advised to use a meter. You'll need to meter the contacts and have someone in the house throw breakers in the panel until you get no reading on the meter for this particular line. I suggest doing this on a weekend where you'll have time to fix everything that's be powered off in trying to locate the proper breaker. When you find the right breaker, label it! Then have someone go into all the rooms in the house to fix alarm clocks, the VCR, the microwave clock, the oven clock, any other timers, etc., etc..
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