I like the idea of the JB Weld / six point socket.

Heat is your friend, plug in the block heater for 30+ minutes, start the car (intake off is OK, just be careful nothing gets sucked in) and get it fully warmed up, and then try loosening the plug. Slowly work it in both directions in small movements, patience is your friend.

If it is still stuck and not yet broken off, I'd remove the injector for that cylinder and rig a small tube to apply penetrating oil to the inside end of the glow plug. I had great success with that method on a plug that was stuck. Give the penetrating oil some time to work, overnight would be best.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC