I just moved into a new house which has the same heaters, and two of them dont work at all. When i opened it up I had the same problem as you, the wires have melted/coroded away. I think i'll just phone S&S Energy to get them to send someone out to fix it. Aparently its because someone at some point would have had the heater on max boost and max temp or maybe covered it up at some point, at least thats what I can see from doing a web search on the problems.
Gary McRae
February 2008