Just anywhere in between the tierod and the suspension piece to force the joint apart (with the nut on of course). You want to apply tension to the ball joint so that it jams up internally and it would give you enough friction to tighten up the nut. As it is now the ball joint is already worn and it just rotating in there.

You sure you cannot whack the 5mm in?

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1992 124 E200 (Euro)