On October 13, 1996, at 1305 eastern daylight time, a Cessna 337G , N20EC, landed gear-up on runway 28 at Fayetteville Regional Airport, in Fayetteville, North Carolina. The personal flight operated under the provisions of Title 14 CFR Part 91 with a flight plan filed. Visual weather conditions prevailed at the time of the accident. The airplane sustained substantial damage, the pilot and passenger were not injured. The flight departed Fayetteville, at 1000. Upon entering the traffic pattern for a full stop landing, at the original destination airport, the pilot placed the landing gear extension handle in the down position, and discovered that the landing gear would not extend. The pilot radioed Fayetteville approach control and advised air traffic controllers of his problem. The pilot flew to Fayetteville and conducted several over-flights to check for the landing gear extension. After three hours of flight, the pilot shut down the front engine and made a gear up landing with the rear engine running. An examination of the aircraft revealed that an unauthorized on/off switch for the landing gear system had been installed under the instrument panel. According to the pilot, when the switch was placed in the on position, the landing gear would retract but would not extend. There were no write-ups in the aircraft maintenance logs or the aircraft checklist that revealed the installation of this switch. Neither the pilot nor the new owner of the airplane were aware of the unauthorized switch installation.