First, check to make sure your trunk seal is in good condition. It is looks okay, you might have a bad rain rail. The rain rail is the piece that connects your soft top to the back of your car. You can check it by pulling back the carpet on your deck lid. You will see that the rain rail is attached around the top of the deck lid with a series of bolts. Have someone pour water on the back of your top while looking at the rain rail where it attaches. If it leaks, there is your problem. A bad rain rail can cause water to leak into the trunk and also onto the deck lid. You may also find water from the deck lid flowing down to the foot well areas.
Another possible is your battery sweating. If your Miata Battery is in the process of dying it sweats - quite a lot. I replaced my trunk seal for $70.00 first, still had lots of water in my trunk. Finally my battery died, I replaced it and the water stopped!
the water temp sensor in the earlier NA models are at the coolant neck in front of the engine. it's the plug pointing straight up on the 90-93s.
Behind the Timing Belt at the front of the engine. The water pump pulley is the top central pulley on the engine. Removing the Timing Belt is required.
You say MX5, not Miata, so do you really mean 2006 or newer MX5? If you really mean 1990-2005 Miata then the water pump is behind the timing belt cover. It is not driven by the t-belt, but the t-belt must be removed to change the pump.
Perhaps a bad door seal? Hole in floor? Rear windshield seal? Trunk seal? (check trunk area for water under spare tire etc.) Perhaps a bad door seal? Hole in floor? Rear windshield seal? Trunk seal? (check trunk area for water under spare tire etc.)
At the rear of the engine, directly in the center, just above the transmission. If you look to the right of your firewall, you can find it by tracing the left heater hose down to the back of the engine. This answer is applicable for 1990-2005 Mazda Miatas.
The life span of any car depends on how well it is maintained and cared for. I have a '94 Mazda Miata that has nearly 200,000 miles on it and have yet to have a major problem with it. Naturally, I have had to change the water pump, spark plugs and a sensor or two, but that is just normal wear and tear. I've read articles of some Miatas going to 400,000 plus miles. So take care of it, and you should get many, many years out of it.
I'm not trying to be a smart aleck here but obviously you have a leak somewhere. It could be anywhere, the roof, the windows, the door seals, the floor pan, the trunk lid, the trunk floor pan, etc. , but if the moisture is restricted to behind the front seats, then I would start looking there.
If it is leaking through the crack next to the soft/hard top just put some sealant there and it should be fine.
they get water when it rains
is the amount of water on earth added to every time it rains
Timning Belt that should be changed every 60,000 miles. (may as well also change the water pump at the same time). http://www.techguys.ca/howto/index.html
it rains