I've been looking for a replacement wiper connector/linkage as well. I need to take it off and see exactly how it's assembled. Mine is split open and there's steel wire wrapped around the shaft and that wire is covered in the black plastic covering.
Here's a bad pic of mine, but you see what's inside, sort of.
I would look into a heavy duty cable type assembly, but you’d only need the sheath. I used Barnett Clutch to build me a custom motorcycle clutch cable and the sheath dimensions were really close to the wiper cable sheath. Might be able to get the right length and ends crimped on?