I have posted something similar to this in the “What have you done to your MX-5 today” thread but I thought it might be useful in this topic area as well.
I have had to constantly unblock the tiny drain holes in the plastic windscreen/scuttle cover trim which would block with the tiniest bit of debris. I carefully removed the scuttle trim on both sides in order to examine the underside of the trims to see the configuration of the drain holes. The existing drain holes have a very short tubular section downwards on the reverse side but this does not connect to anything and stops just short of the metalwork. This probably does not help the fact it blocks so easily, To improve flow I drilled a same sized hole (5mm diameter) at 7mm distance (centres) from the original holes (towards the centre of the car). I then used a modelling knife to cut through the plastic between the holes and form a slot. When this is done you will necessarily cut half way into the tubular section on the underside but I would not think that this particularly matters. This could be done with the trims in situ but I found it much easier to finally neaten up the holes using swiss files with the trims removed from the car. The result can be seen below.
ND owners may also be very pleased to know that the plastic inserts for the fasteners that hold the trims down do not penetrate the scuttle metalwork and hence there will be no more leaks into the footwells of the car as suffered by previous model owners (including my NC).
