There are a couple of known leak points with a Miata; the two convertible top drain lines and the two windshield cowl screws and I knew this prior to buying the car. I cleaned the drain lines for the top as soon as I got home from buying the car and did the windshield cowl screw fix a few days later. I disassembled the cowl and cleaned around the plastic bracket before applying silicone sealant. I put it all back together and called it a day feeling good about my fix but knowing I wouldn't be sure until the next rain.
It wasn't long before we had one of our torrential Summer downpours. Once the shower had past I ran outside to check the Miata. It's parked outside because we only have a two-car garage. The driver's side was completely dry so I thought I was in the clear. I then checked the passenger side and my heart sank. The carpet was wet. Not soaked, but wet nonetheless. I knew the top drain lines were good so it had to be the windshield cowl screw on the passenger side.
This second time I took a different approach (and my time). I'm not known for being a patient person after all. I pulled the car into the garage so that I could start drying the interior while trying to fix the passenger side again. This time I removed the white bracket completely and cleaned it and the spot in the body where it plugs into with rubbing alcohol. Once the alcohol dried I applied RTV sealant - and a lot of it. Here's where the patience came in. I let the RTV fully cure over a couple of days before putting the cowl back in place.
We've had many rains both heavy and light and the interior has stayed bone dry! Here's a video that I used for reference for others needing the same fix.