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.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.