THE PIPER PA-31T WAS A FOREIGN REGISTERED ACFT (FEDERAL REPUBLIC OF GERMANY). THE OWNER/PLT IN THE LEFT FRONT SEAT HELD A GERMAN COMMERCIAL CERTIFICATE WITH AIRPLANE INSTRUMENT, SINGLE ENG LAND & MULTI-ENG LAND RATINGS. THE PLT IN THE RIGHT FRONT SEAT HELD AN AMERICAN PRIVATE CERTIFICATE WITH AN AIRPLANE SINGLE ENG LAND RATING. DURING ARRIVAL, THE ACFT WAS CLEARED TO ENTER A RIGHT TRAFFIC PATTERN FOR RWY 32R & WAS TO FOLLOW A DECATHLON THAT WAS LANDING ON THE SAME RWY. WHEN THE PA-31 AIRCREW CALLED TURNING ONTO A BASE LEG, THE TOWER ASKED IF THEY HAD THE DECATHLON IN SIGHT, BUT THEY DID NOT REPLY. THE DECATHLON PLT WAS THEN INSTRUCTED TO CHANGE HIS APCH TO 32L & WAS CLEARED TO LAND. THE PA-31 WAS OBSERVED TO OVERSHOOT THE TURN TO THE FINAL APCH COURSE TO RWY 32R. WITNESSES REPORTED THE ACFT MADEAN ERRATIC, SLOW SPEED, NOSE HI TURN AT LOW ALT, THEN IT ENTERED A RIGHT TURN, NOSE DOWN SPIN, CRASHED & BURNED. IMPACT OCCURRED ON A GROUND STRUCTURE. AN INVESTIGATION REVEALED NO PREIMPACT/PART FAILURE OR MALFUNCTION. FIRE DMG TO 5 AUTOS.