When the aircraft was about 1 1/2 miles from the destination, a fire started on the left side of the floorboard against the firewall. The pilot turned off the electrical system, and before landing, shut off the fuel selector. After landing, the occupants egressed and the plane continued to burn until fire department personnel arrived and applied dry chemicals. During an investigation, a fuel line was found with areas of thinning. This line was routed from the firewall to the fuel shutoff valve on the left side of the cabin. This area had been exposed to high heat during the fire. A possible source of ignition was a power supply for the radio, which was attached to the firewall under the instrument panel just above the fuel line. An internal examination of the power supply unit showed an area of intensive heat around the relay assembly. According to an engineering report, the wall thinning of the fuel line (made from aluminum alloy 3003) was a result of loss of material at the inner surface.