The bulb size was a #921 12V 18W.
These are the ones I got on Amazon - https://amzn.to/3LjcaBv
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This was for my top center third brake light on my 97 Toyota 4runner. I am guessing it would be the same for all 3rd Gen 4runners, but I am not really sure. I also saw it mentioned when I bought the new bulb that is can be called a T15. But I have not verified that. Hope this helps.