VEHICLE WAS BEING OPERATED ON A PAVED ROAD WHEN THE REAR WINDOW DEFROSTER WAS TURNED ON TO CLEAR SNOW ON THE OUTSIDE. SEVERAL MOMENTS LATER I HEARD A HISSING SOUND COMING FROM THE REAR INTERIOR OF THE PICKUP CAB AND THEN LOOKED INTO THE REAR VIEW MIRROR, AT WHICH TIME I SAW SMOKE EMITTING FORM THE LOWER CORNER OF THE PASSENGER SIDE FIXED WINDOW PANE (THIS IS 3 PIECE REAR WINDOW WHERE THE MIDDLE PAN OPENS VIA ELECTRICAL SWITCH). THE HISSING CONTINUED WITH THE SMOKE THAT WAS COMING FROM THE LOWER CORNER OF THE WINDOW NEAREST TO THE CENTRAL WINDOW PANEL. I IMMEDIATELY RECOGNIZED THE ODOR OF AN ELECTRICAL FIRE (BURNING INSULATION) AND THEN TURNED OFF THE REAR WINDOW DEFROSTER SWITCH. THE HISSING SOUND AND SMOKE CEASED. A FEW SECONDS LATER 8 HEARD A LOUD 'POP' SOUND COME FROM THE BACK WINDOW AND OBSERVED THAT THE ENTIRE REAR PASSENGER SIDE WINDOW HAD SHATTERED, IN PLACE WITH THE FRACTURE LINE EMANATING OUTWARD FROM HE AREA WHERE THE SMOKE WAS FIRST OBSERVED.