If you've verified that it's the tank and not the equally problematic cap (make sure you have the latest version), this post might be helpful:
Coolant expansion tank replacement
And invest in a Bentley manual. It's the best $63 you'll spend on your car. This price is the best you're going to find for a new one:
http://www.renntech.org/forums/index.php?app=spoffers&do=amazon