Big Al,
I just replaced the whole thing. Ford calls it a cable assembly. It includes the cables and the plastic cylinder that fits over the lock. You basically have to break the plastic cylinder apart to get it off of the lock. I drilled the plastic rivets off to do it. Oh yeah, I had to crawl into the bed, under the cover, and pull on the uncovered section of the cable (at each latch) to release the latches so I could get the cover up.
Then it's just a matter of threading cables and attaching the new cylinder to the lock. Take a look at the inside of the cylinder and the outside of the lock. There's a wide indent/detent, and a narrow indent/detent on opposite sides of the plastic cylinder and the lock - meaning there's only one way that they'll fit together.
Cost was about $40 at the Ford dealer here in Charleston, SC.