The car leaks badly under the dash and has always put me off the rain. And before anyone says....no it can’t be cured unless Morgan make a new body shell for me! And of course I now live in a rainy part of the country!!
I can't believe that can't be cured. One way or the other it should be possible to track it down. There are only so many possibilities.
Windscreen seal, and or windscreen heater wire hole,
windscreen mounting bolts,
bonnet hinge fixing screws,
wiper spindles,
washer jets,
heater box,
then under the bonnet:
heater box drain which spills onto the panel above the footwell and from there can enter through various fixing points, including under the ECU box,
water getting under the ECU box and passing the rubber wiring grommets.
Perhaps the entry point can be found by sitting the car under a garden sprinkler after putting paper towel in a lot of strategic points and then checking to see which is wet. The dash could stay in for this but with the lower plastic cover removed to get access. Of course the garden sprinkler idea won't work if there needs to be wind pressure to force the water in somewhere. In that case a drive in the rain with the paper towels in place may produce a result.