We had this same issue on our 2002 Islander.
The design was poor. The yellow handle on the sea cock has a fitting through the hole in the end, and the fitting has a ball on the outside end. The handle has a socket at the bottom, and it fits over the ball, allowing the entire thing to pivot when the rod is moved up and down. Problem was with ferrous metals that rusted away, until failure of the connection.
Contact GW customer service to see what they can or will do to assist you. I have heard of folks getting the new, redesigned rods for free, or for a reasonable cost.
As to actually getting the work done, it was very difficult. I had to removed all the batteries from the shelf under the rear seat, then remove the panel that the battery switches are mounted on, just so I could get my upper body down into that area.
It was NOT possible to remove the fitting in the hole in the yellow handle while upside down, and working with one hand. I found it much easier to removed the nut and washer that hold the yellow handle on the seacock, then take the assembly home where I could put the handle in a vise on the workbench. One that was done, it was easy to connect the new reach rod, then back to the boat to reinstall the yellow handles on the seacocks.
Good Luck!
Brian