Modifying widget content via plugin?

Hello,

I have written a plugin that finds and replaces a string with a special drop down box in a posts or pages content. I used the ‘the_content’ action/filter/hook to do this. It is working great.

I would also like this plugin to affect text based widgets created by the site owner. These do not use the_content hook. What hook do they use? How can I get my find/replace plugin to effect the content of text widgets?

Thanks