I recently bought an Amiga mouse on Ebay. Unfortunately the little sucker wouldn't work in the horizontal axis. After a little searching I found this very useful youtube video. I plugged the mouse into my scope and saw right away that one of the photo transistors wasn't working properly. So I decided to just replace them.
Taking the black body off was a pain in the bottom. I DO NOT recommend trying to lever the locking parts apart. Just try to lever the whole part up. I broke mine and had to glue them up.
You need to put the parts together and slide them in, just really gently. The slide the wheel in really gently as well.