I do remember about 8 or 9 years ago my grandparents lived in a 1981 mobile home with
an electric (Coleman) heat/AC unit and it had
the floor registers. Water went through their ducts too. They said a part of the condenser
(I think)rusted through or corroded and had to be
replaced. Hope this may give you some idea of where to begin with the furnace:) Good Luck!