Another method to consider.... try grinding the head off the bolt with a dremel. Once the water pump slips off you can perhaps deal with the stuck bolt a little better since there's some length to work with.

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95 E320 Cabriolet, 169K

If you can pound a 12 point 12mm socket onto the head of the stripped out allen bolt you will be able to loosen it, it will grab the outside of the head goodentight JA.

A 3/8 drive would be best...............

IF that does not work, try prying the water pump away from the block and rotate it at the same time, this action will loosen the bolt.....!!