A strange one this. I've stripped the interior of my Rossa. It was pretty damp. Door seals need doing. However, when I removed the n/s seat belt mounting bolt (near the floor by the sill), water came gushing out at quite a rate. Its as if the water had been standing in the sill cavity and had slowly filled up over the years. Has anyone else had this. Should there be a drain for the sill cavity and if so where?

You've found a good way to save some weight!

I had exactly the same issue but only on the side that had the header tank in it. I know the previous owner had overfilled it with water and it had once boiled out of the overflow into there so I’ve put it down to that and haven’t drilled any extra holes, but intend to monitor it. This probably doesn’t help your question, but thought I'd mention it.