On October 13, 1995, about 2010 eastern daylight time, a Piper PA-23, N65SC, piloted by the owner/operator, experienced an in-flight fire while operating in Instrument Meteorological Conditions (IMC) in cruise flight in the vicinity of Beckley, West Virginia. The fire was extinguished in flight, and the airplane landed at Beckley, West Virginia, without further incident. The airplane sustained minor damage. The commercial pilot and the three passengers reported no injuries. Night visual meteorological conditions prevailed at the destination airport, and the flight operated on an IFR flight plan. The personal flight operated under 14 CFR Part 91, and originated from York, Pennsylvania, at approximately 1830. The intended destination was Beckley, West Virginia. The pilot stated that the flight proceeded normally until the airplane was about 45 miles from the destination airport. He reported that the airplane was in level flight at 8,000 feet Mean Sea Level (MSL), in clouds with light precipitation, when there was "...a sudden flash (spark)..." and then a fire behind the right side of the instrument panel. The pilot stated that he declared an emergency with Air Traffic Control (ATC), then turned the master switch off in an attempt to "...stop the source of the fire." The pilot reported that the fire grew and the cabin quickly filled with smoke. He stated that the right front seat passenger moved to a rear seat, away from the fire, and they were able to extinguish part of the fire with the passenger's coat. The pilot reported that the fire gradually "...went out, mostly on it's own..." and he made an uneventful landing at Beckley. Maintenance personnel and Federal Aviation Administration Inspectors performed a postaccident examination of the airplane. They reported that the fire damage precluded a positive determination of the origin of the fire.