One Wire Bad on a Slick Harness

Do you have to replace the entire harness or can you replace the wire?

I've replaced the plugs on the left rear cylinder and checked to make sure
no induction leak. The egt on no. 2 cylinder is running 200 degrees low.
The other 5 cylinders are fine.

Mag check shows the problem clears up on the top plugs but that one #2
cylinder misses (shown by low egt and rough engine) on the left mag firing
the lower plugs.

I'm going to look today to make sure the valves are opening and closing.