Ken is right. It almost always comes from the entry of the rudder post. Problem with that foam - once it is wet it never dries out. Needs to be dug out, drilled out, then dried out and refoamed using high density two part foam. Then the skin can be repaired and the issue at the rudder post addressed. Unless the stainless webbing inside has corroded at the welds, there is no reason you cannot do the rebuild yourself.