Looking at the location photo in the book of the drain and cleanout is unclear. Supposed to be beneath the top behind the passenger seat.
(2016 MX5 Touring)
Anyone know how to access it?

 

Okay, so, it's actually pretty easy. Basically, you pop up the trim over the threshold, then take off the panel on the back, in front of it. This is loads easier than trying to reach down and around.
There's lots of videos out there, but I like this one:
Pay attention to the angle that the foam under the basket goes in, and be sure to snap the basket back in. Also, look at it from all angles before you start so you can see what it's supposed to look like after you finish.

 

1. Thanks for the link.