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I found that these typically are removed with the rear wire harness and stay with it. You may see one on a harness on ebay. I'll keep an eye out for one.

Sorry, I got mixed up.  You don't want the plug for the blocked hole, you want the one for where your license plate wire goes to the bumper area (which is same as NA picture I showed above) right?

The grommet on a North American car has wires running through it.. so it has a hole.... not what you need:

Here you go... at home for you too:

https://www.ebay.com/itm/9mm-3-8-10-PCS-RUBBER-SNAP-IN-BLANKING-PLUG-BODY-HOLE-BUNGS-STOPPER-GROMMETS/162828533005?epid=518240321&hash=item25e9563d0d:g:70QAAOxyeIdTFNgz

I found that the radio antenna delete cover was just a plain old  rubber grommet that is found elsewhere on the Z (a common floor drain plug grommet I believe).

Perhaps you can measure the opening hole of the license plate hole and find an appropriate grommet from a different Nissan car of the same era  or....... if lucky, the same size hole and grommet is found elsewhere on a Z?