What was found
AT FL400 WE TURNED THE DEFOG FAN ON & SMELLED AN ELEC OR PLASTIC BURNING SMELL. WE TURNED THE DEFOG FAN OFF IMMEDIATELY & THE SMELL DISSIPATED. REMOVED FLOOR PANELS & INSPECTED DEFOG BLOWER & ALL ASSOCIATED DUCT WORK IAW AMM 21-21-01, NO DEFECTS NOTED. PERFORMED TEST OF CABIN ENVIRONMENTAL SYS WITH ENGS RUNNING GREATER THAN 75% N2 FOR 30 MINUTES IAW AMM 21-00-00. DURING THIS TIME WE SWITCHED BETWEEN NORMAL, LT & RT. WE SELECTED ALL TEMP RANGES IN BOTH AUTO & MANUAL MODES. DURING THIS TIME WE ALSO CONTINUOUSLY SWITCHED THE DEFOG FAN BETWEEN HI, LO & OFF, NEVER DURING THESE RUNS DID WE NOTICE ANY BURNING PLASTIC OR ELEC ODOR. REMOVED PN 5703-4 & SN 1518. INSTALLED NEW COCKPIT DEFOGGER FAN PN 9918002-1, BC64A-2, SN 399 IAW AMM 21-21-01. ALL CHECKED GOOD.