Water gathers in the trunk. How to take care of this problem?
Simple , find the leak with a garden hose then fix it. Or have a body shop solve this for you.
This video shows you what is happing and why water is getting into your trunk. In the drawing in the video, he shows you that he puts a wire mesh in the hole in the trunk. you probably have a clog in the tube below that. that tube needs to be cleared. then if you want to put a mesh like he did you could. this would help block debris from clogging it again. but you went this many years before it clogged. I do not think you will have the car long enough that it warrants the mesh being added.
Informative vdo, and very likely cause of the wet trunk. Many car designs route the rear-windshield rain-water over the tail-lights to ground. If tail lights springs a leak, same effect. But it seems the Miata has non-tail-light path to ground. I’m a little dubious of the merits of installing mesh however. Will have to be frequently cleared.
I vacuum the front and rear drains at every engine oil change, good opportunity while the oil drains. My vehicles would have improved rain-water-resistance had I discovered that technique earlier.