On April 26, 1995, about 1109 central daylight time, a Beech S-35, N67HC, developed a cockpit fire during an approach to the Huntsville International-Jones Airport, Huntsville, Alabama. The airplane was operated by CPT INC., under the provisions of 14 CFR Part 91, and instrument flight rules (IFR). An IFR flight plan was filed for the personal flight. Visual meteorological conditions prevailed. There were no injuries to the airline transport pilot/certificated flight instructor, and minor injuries to the private pilot. The airplane was substantially damaged by the fire. Origination of the flight was Memphis, Tennessee, about 0730, on the same day, with an enroute stop at Nashville, Tennessee. According to the pilot, an instrument approach was being conducted to Huntsville. About seven miles from the airport, while descending to intercept the runway 18L, instrument landing system glide slope, white smoke was observed coming from the top of the glare shield, just left of center. The pilot attempted to contact the tower, however, the tower did not receive his transmission. Shortly thereafter, black smoke began coming from under the instrument panel, and flames could be seen through the production split between the left and center instrument panels. The approach was continued to a landing. After landing, the airplane was steered off of the runway and both pilots exited the airplane. The aircraft was equipped with oil and fuel pressure lines from the engine compartment to gages installed in the instrument panel. Aircraft records show the aircraft received maintenance work two days prior to the accident to install a new autopilot system in the center instrument panel. During the maintenance work, the old autopilot was removed. Components of the autopilot were removed from behind the instrument panel. An examination of the airplane was conducted by an airworthiness inspector from the Birmingham Flight Standards District Office. He reported that dead end wires were left installed behind the instrument panel when equipment had been removed. The avionics bus relay was mounted inside the cockpit on the lower firewall, about four inches below the oil line. There was no protective covering for the relay. The oil line directly above the avionics bus relay was burned away, and there was evidence of extreme heat in the same area. The inspector stated that the avionics bus relay carried sufficient amperage to arc and burn a hole in the oil line, igniting the oil.