You need to get the taper to grip in the hole before you get to the locking nylon part of the nylock. Just put about 5 or 6 washers between the nut and the wishbone and tighten it up enough for the taper to grip. Then back of the nut, remove the washers and you should be able to tighten the nylock without the taper rotating.
Or, if it's the lower wishbone just put a jack under the ball joint to put a bit of pressure on the taper.
...peter
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