There is a connector for the back-up lights on the upper driver's side of the transmission case. If you reach down just below the baffle that connects the airbox to the throttle body, you should see it in that vacinity. Try unplugging and reconnecting the connector to see if that may solve the problem. If not, disconnect the connector from the transmission case, pull the the rubber boot upwards and inspect the two wires going to the connector.
My friend had the same problem and it turned out to be one of the wires breaking off from the connector. He soldered it back on and the problem fixed.