What Dave said - if your car didn't get the warranty fix that they were doing about eight or nine years ago, you're going to get that corrosion from the inside out. If you have a '96 GTS in nice condition, you might want to think about chasing down some of the aftermarket side sills, have them painted to match, and put your stockers in storage. That will be much less hassle than gutting out the stockers, doing aluminum repair, repainting, and then risking it happening again (probably won't if it's fixed right, but it only takes a pinhole sized area of oxidation in aluminum to eventually eat its way all the way through).
