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VZ Executive lights are the same size as;
VY: Berlina, Calais, HSV
VZ: Acclaim, Lumina, SVZ, Berlina, Calais, HSV
VY Executive lights (smaller) are the same size as,
VY: Acclaim, S, SV8, SS
VZ: SV6, SV8, SS, SSZ
There is a good inch and a half difference in height, and to put smaller (SS) lights in your car, you would need to change the rear bumper. I doubt whether the loom would need to be changed.
Also worth noting all after market tail lights are the smaller SS size lights. They won't fit your Executive.