What was found
TWO POWER LAMPHOLDER ASSEMBLIES (BALLAST) WERE BURNED BY ARCING WITH THE ELECTRIC WIRE CONNECTOR WHICH WAS FABRICATED AND ASSEMBLED BY BRUCE INDUSTRIES (FAA PMA 400-067), AND DELIVERED TO AEROMEXICO FOR INTERIOR MODIFICATION OF DC9'S. THIS AIRCRAFT WAS MODIFIED ON 3-10-98; HENCE, THE PARTS WERE ALMOST NEW (6 MONTHS OF USE SINCE INSTALLED). SUBMITTER STATED, APPARENTLY, THE PLASTIC COVER AT THE CONNECTION WAS MELTED BY OVERHEAT AND THEN THERE WAS A SHORT CIRCUIT THAT BURNED THE BALLAST.