I had this problem and found that the only way I could get the bolt out (drilling wouldn't work without removing the a/c condenser as well as the radiator) was to remove the fan bearing bracket, with fan attached (which was enormously difficult, I recall having to cut one or two of the bolts with a hacksaw blade because they were too long to remove with the fan still on) and then putting the fan bearing bracket in a vise (gently) and using a small cold chisel to loosen the bolt. NIGHTMARE!

Only done it once though, there may be an easier way. Oh, and I replaced the bolt with a high tensile one which makes it a lot more resiliant to stripping.

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Andrew
300e 294,000mi
380sel 185,998mi
380se 309,980mi