I had a similar experience recently with an SUV.

I "carefully" drilled a small hole in the center, then used an EZ-Out to get the loosening process started. If you damage the threads, some sort of repair(depending on severity) will be required.

To me the trick here is accessibility....can you get a straight shot at it from underneath. If not, pull the radiator.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"