Thought I would add my 2c to this for any people who have this problem still/future. I've sold my S2000 but did fix this before new owner purchased.
I was having the same issue tail light would stay on very dim, installed a 4th brake light to act as a resistor in boot which worked for awhile before burning a hole in my mates jacket lol! I also ran into a problem where after the bulb would get a couple weeks old it would start drawing too much current and my brake lights wouldn't work at all some times so I'd have to unplug globe when driving then plug back in when parked which started to annoy me.
So I fixed this in 30mins, Purchased a LED load resistor from supercheap which acts as a 21w globes (perfect) this simply wires to the positive and negative of the tail lights which the 4th globe was wired to.
Here is a picture of the resistor form Supercheap.
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