A PIPER PA 32R-300 LANDED SHORT OF THE RWY AFTER A LOSS OF POWER. THE PLT REPORTED DURING THE CLIMB THAT HE TURNED ON THE HEATER AND THE WINDSHIELD TURNED BLACK OBSCURING OUTSIDE VISION. THE PLT ALSO INDICATED THAT THERE WAS A 'PUNGENT ODOR' AT THE SAME TIME. THE PLT RETARDED THE THROTTLE AND THE WINDSHIELD BEGAN TO CLEAR. WHEN THE PLT ADVANCED THE THROTTLE TO RESUME THE CLIMB THE ENG BEGAN TO RUN ROUGH AND THEN WOULD NOT PRODUCE POWER. HE WAS GIVEN RADAR VECTORS TOWARD AN AIRPORT. THE PLT INDICATED THAT HE WAS UNABLE TO JUDGE HIS POSITION IN RELATION TO THE AIRPORT DUE TO POOR VISIBILITY. HE PREMATURELY LOST ALTITUDE BY S-TURNING AND SUBSEQUENTLY LANDED SHORT OF AND PARALLEL TO THE RWY. THE AIRCRAFT'S ENG WAS EXAMINED UNDER THE SUPERVISION OF A FAA AIRWORTHINESS INSPECTOR. IT WAS NOTED THAT 4 SPARK PLUGS HAD BEEN DAMAGED IN THE AREA OF THE ELECTRODE. FURTHER EXAMINATION DURING A TEARDOWN INSPECTION REVEALED THE MUFFLER HAD EXPLODED BLOCKING THE AIR INDUCTION SYSTEM.