I don't think they are, but I tried to google it up and couldn't find anything. Are the S13 rear seats interchangeable with the S14 rear seats?

 

No, not in the same sense the front ones are.

I DO know people running S14 rears in their 13's, but it's not a direct bolt-in affair, it requires a lot of work.

I can point you in the direction of someone who's done it, if you're up for a project. PM me if interested.