To find the drain tube, open the hood of your Miata and look on the passenger side of the engine bay. You should see a small, black rubber hose running from the air conditioning unit to the ground. If the hose is not visible, you may need to remove the plastic cowl that covers the engine to find it.
If the drain tube is clogged, it can cause water to back up into the air conditioning unit, which can damage the unit. If you think the drain tube is clogged, you can clean it out using a pipe cleaner or a piece of wire.