#3 … That other part that looks like it might be it (#4) is actually the rubber seal that goes under it, next to the body of the car.
More ammunition for getting a vernier 
All cars are not equal.
The pic is my 1998 x308. The SNGB parts list I linked to shows the same OEM fitting for all x300 models. Item 4 is a grommet not a nut and has no flats. With respect you may have a non OEM fitting.
E bay shows aerials that are “suitable for X300” and have a hex nut where SNGB shows a grommet - these may be “suitable” but are not OEM. If yours is not OEM having others measure theirs will not help I’m afraid.
Frankie
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Sooo, was this situation resolved?
Not for the moment, have more stumpers working on right now. As mentioned, the cig lighter no longer has power. I ordered a NOS element, and first time I pushed it down while in the “hole”, the “feel” was correct, yet - nothing ! 
I checked the 25-amp fuse (#5 in the boot box), and it is fine. wth??? 
What else can possibly be amiss with a simple cig lighter if the fuse and element are both good, esp. on a '00 MY??? 
(and please don’t tell me it’s the unprogrammed key issue again - IIRC, the cig lighter in the XJ8s even has power going to it w/o the key in the ignition … 
)
Anyone? I hate to start a new thread just over this issue … 
The ignition must be in position 1.
Frankie
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k, and thanks FrankE. Well maybe that’s the rub right there … again, it’s in pos. 1 but w/o a programmed key maybe it’s just not letting power get to it, desp. being in that position. 
Friend used his wrenches on it for size, and he reports it is a 14-mm. I don’t see how, when I tried my wrenches it was too big for the 16-mm., the largest in my set, as mentioned earlier. Strange. 