AS THE ACFT REACHED 15,500 FT, APRX 30 MI FROM THE DEP ARPT, THE PLT & PAX NOTED FUMES THAT IRRITATED THE EYES & CAUSED DIZZINESS & BREATHING DIFFICULTIES. THE PLT REVERSED COURSE, THEN ELECTED TO MAKE AN EMERG LNDG ON A DIRT ROAD. DRG THE LNDG ROLL-OUT, THE LEFT WING STRUCK 1 OR MORE METAL POSTS. WHEN THE PLT & PAX EVACUATED THRU THE PLT'S DOOR, FLAMES WERE OBSVD EMINATING FORM THE AFT PORTION OF THE LFT WING & THE LFT MAIN TIRE & WHL WERE BURNING. THE FIRE HAD SELF-EXTINGUISHED PRIOR TO THE ARRIVAL OF EMERG EQUIPMENT. AN EXAMINATION REVEALED SVRL BURNED ELEC WIRES UNDER THE LWR LFT INST PANEL. THE BURNED WIRES WERE CNCTD THE STROBE LGT, SFC DE-ICER & WINDSHIELD HEAT OR PROP DE-ICE SWS, AS WELL AS 3 UNUSED WIRES. THE SOURCE OF THE ELEC FIRE WAS NOT DTRMD. NO DIRECT RELATIONSHIP WAS FOUND BETWEEN THE BURNED WIRING IN THE COCKPIT AND THE FIRE ON THE LEFT WING & GEAR. A DENT WITH CHORDWISE GASHES WAS FOUND ON THE LEFT WING. FUEL STAINS SURROUNDED THE GASHES.