Two problems with this car , the core plugs in the top of the engine sometimes leak , but then you say rainwater is in there so that is not it , so its the second one , if you look at the washer jets on the top of the bonnet you will probably see no seal between the bonnet skin and the jet itself , Ford omitted this on early cars as they did not think it was needed , we have swapped loads of the early jets for the later rubber sealed jets and this cures the problem .