We suspected the rear light was leaking, this is possible if the rubber seal around it has perished.
However, upon inspection we found that at some point this car has had some bodywork and holes have been drilled through the back of the panel and is letting in water.
This is not a common problem on MX5s (obviously) and requires the intervention of a panel-beater or electrician to occur. This is why when cars come to us, we ask if the car has had previous bodywork or been involved in an accident. A shunt in the back can cause all sorts of panels to go out of alignment, or the sealant on seams to become cracked... or a panel beater could have drilled a bunch of holes or decided to leave some parts out because he decided they weren't important. Car audio specialists are another culprit and have a nasty habit of making holes where holes don't belong.
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