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After a cold night, there's always condensation on the underside of the boot lid. And the water drips onto the boot mats and it's quite damp. Is there anyway to stop this?

move to some where that is warmer

or park in a garage or shed

lol

Is that a cause of some part that's come loose or something mechanically not right?

zephyrus17 wrote:Is that a cause of some part that's come loose or something mechanically not right?

There are drain holes in the bottom of the boot; there are also seals around the lights. If either are missing or damaged then water can get into the boot.

Once the carpet is wet there is a cycle. Wet boot floor means moisture in the air to condense on the boot lid. Condensation then drips into the carpet. Cycle starts again.

I had to get a blow dryer onto the boot of my old NA because of this. I ended up removing the carpet to make drying easier. Once the carpet is dry you can put it back in the boot (which hopefully is also dry by now because you've found the hole and plugged it).

HTH
Boags

Yes, you're boot should not be wet/condensation. Find the leak and dry it.

Ok. Thanks. I'll look into it tonight.

& if heose drain holes are OK I'd also be looking at the tail light seals. Of course they only leak if water runs over them or it has to be raining for them to leak.

Check the drains behind the seats
Have a read HEREand HERE
Dependant on what angle the car is parked, when these back up it has been known for the water to flow into the boot.

Will it cause rusting problems?

zephyrus17 wrote:Will it cause rusting problems?

Leave it long enough without fixing - Yes

I lifted the carpet (which is being air dried now) and couldn't find any apparent drain holes. I see two holes in the central depression, but they are blocked by two rubber tabs. Or are they under the carjack?

There should be no drain holes in your boot (that I can remember).

There are vents for your battery but they should have tubes running to them.

Hmm it's ok. I wiped the underside of the boot as dry as I could. I also had a look at the metal under the mats behind the seats. It was quite damp, but nothing serious. There's a bit of rust at spots, but nothing too bad.

Those mats should be 100% dry. Sounds like your leak could be from the rain rail that acts as a gutter at the bottom of the softtop and channels water towards the drain holes behind the seat-belt tower. I'd suggest you take out the carpets and mats from the rear parcel shelf and give them a few days in the sun to fully dry out. While they're out, put your softtop up and lightly hose the outer surface, gradually moving the hose around the softtop, and look for leaks on the inside of the car. If you find any, then your next step will be to remove the rain rail, patch it up with some gaffer tape (see Miata.net for instructions), and re-install.

BTW, do you park on a steeply sloping surface? Normally a slight gradient is fine, but if the front of the car is significantly higher than the rear, there is potential for the rain rail and drains to be angled such that water cannot drain out properly. This is something you can check easily in the above test too.

Very good advice Guran