I've wanted to do this too and it seems to work well by adding the following at the bottom of
auth.php
Code:
if($active_modules["Bestsellers"])
@include_once "../modules/Bestsellers/bestsellers.php";
I actually haven't had any success with it showing up while browsing categories though, and it didn't make a difference by adding the include to every file.
edit: This should have been obvious before, but I just realized it works only if there's ordered products within the categories you're browsing.