In the many block heater threads I've seen, they have all agreed on the plug being a bugger to get out, and advised on a few days of penetrating oil before hand, as well as possibly requiring up to a 5' (yes, foot...) breaker bar to get them out.

At least one person mentioned being very careful in how you have the car supported, as he was worried about twisting hard enough to push the car off of jack stands.

Best of luck with it!

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...