Re: What would cause it?
Disconnect the Positive lead from the battery.
Using an Ohmmeter, attach one lead to ground and check the positive lead on all components that use the suspected circuit one at a time. Once you find the lead that is grounded, go to where it plugs into the main harness. Disconnect the plug and check that same wire on the main harnesses plug. If you don't have continuity anymore, the problem is between the component and the main harness. If it still has continuity, you must follow the wire to the next connector. Keep doing this until you find the short.