Originally Posted by dsbrantjr

caunyd: Don't skimp on new cables, use a good quality set of cable wires and housings. Stainless steel is a must and die-drawn inner wires will run much smoother, as will lined housings. Be sure to properly prepare the cable housing ends after cutting (with proper cable cutters); grind or file them flat, use a pointed object to ream open the inner liner and use the appropriate ferrules on the finished ends.
Here's a link to the shifter assembly drawing:
http://techdocs.shimano.com/media/te...9830630796.pdf
It looks like you remove 2 screws and pop off the top cover and mechanism is right there. It should be clear how to replace the wire. AS dabac said, you might not even have to open it that far.