I've had great success removing broken bolts and studs using left hand drill bits. The combination of heat from drilling and the left rotation means the broken piece is usually out before I have enough depth to insert an extractor.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/showthread.php?t=296386