APRX 15 MIN AFT TKOF, LGT ICG WAS NOTED ON THE WINDSHIELD. THE AIRCREW ACTIVATED THE WINDSHIELD WASHER/DEICE SYS WHICH USED ISOPROPYL ALCOHOL. HOWEVER, ONLY A LITTLE DEICING FLUID WAS NOTED ON EITHER SIDE. THE SYS WAS ACTIVATED AGAIN. AFT HLDG THE SW SVRL SECONDS, THE ODOR OF ALCOHOL WAS NOTED.DEICING PROC WAS STOPPED, BUT SHORTLY THEREAFTER, GRAY-WHITESMOKE BGN COMING FM BLO THE FLOOR. THE AIRCREW BGN DIVERTING. THICK SMOKE FILLED THE CABIN & FIRE BROKE OUT ON THE FLOORBTN THE PLTS AS THE ACFT DSCNDD BLO THE CLDS. THE CO-PLT TRIED TO USE 1 FIRE EXTINGUISHER, BUT IT WAS TOO HOT. NO ONE ATMTD TO USE THE CABIN EXTINGUISHER. CRSH LNDG WAS MADE ON A FRZN LAKE. ALL BUT 1 OCCUPANT WERE EVCUATED BFR THE PLANE WAS DESTROYED BY FIRE. AN INVESTIGATION REVEALED TYGON TUBING WAS USED IN THE DEICE SYS. AFT CONTACT WITH ALCOHOL, THE TUBING HARDENS, BCMS MISHAPED AT CONNECTION POINTS & OFTEN RESULTED IN LEAKS. LEAKS WERE REPAIRED BY RMVG ENDS & REATTACHING. A REPAIR WAS MADE ON 2/18/82 IAW DEHAVILLAND PROC. FIRE EXT LCTNS NOT SUF MARKED OR NOTED ON SEATBACK SAFETY CRDS.