[kh99]

Well, if you're going to add it to the code instead of making a plugin (which is easy, if you want to try), then it should go in includes/functions_search.php. That is if you want all the similar threads to have more than one post.

But if you meant that you only want to have a similar threads section displayed on threads that have more than one post, then you probably would do that in showthread.php.