I have the electrochromic outside, driver's side mirror on my 2001 that has suddenly discolored. I've heard that the cause is a broken seal that holds the fluid in, and that if I don't replace it, this fluid will eat the paint. Only problem is, I can't find anyone that has the mirror glass with the electrochromic option. The whole outside mirror is available, but at a dealer list of >$700 (plus having to paint it to match), that's not a viable option - even at GM Parts House, it's still $545.
The plain glass is available for ~$60, and I've even seen glass with heaters and an aftermarket glass with turn signal, but no one seems to have the electrochromic glass only.
Problems with replacing driver's side outdoor mirror
I have had same problem with chromatic mirror leak as shown in other posts. PArt number for my C5 2003 is 12455610. Good luck this time and previously with GMpartdirect.com but $164. Real problem is installation. Actuator pins are not just rounded but have wings on ball head!!!! So if do not line these up with grooves in receiving holes then no amount of force will get the pin in !!
When doing this blind by pushing on mirror face, it drove me crazy. Had to do as suggested earlier in this thread of removing actuator pin and installing in mirror back before installing. Problem here with seals but never could get around problem in any other manner. Best of luck for next person with this problem.