Multiple Category Drop Down Boxes to Search Posts

I’m working on a project that has a listing of workshops. Categories in this case might include things like the artist giving the workshop, the location, the year, the topic, and etc.

What I’m trying to do is put together a series of drop down boxes that will allow you to filter the posts: Artist #1 is giving a workshop in 2010 in Arizona, for example.

I’ve found the multiple-category-selection-widget which is PERFECT for this, but it seems to be broken in WordPress 2.8.4.

I’ve tried doing custom search forms that have the drop down boxes, but when the search query comes up, we have a format like this: /?s=&cat=13,17 – This query brings up all posts in categories 13 and 17, not just the posts that are in BOTH 13 and 17.

Does anyone have a method or any advice on what to do about this problem? I need something that will let me do the search based on posts that are ONLY IN the selected categories, not ALL the posts that are in ALL the selected categories.

I’ve seen a lot of other people trying to do this, but I haven’t really seen a solution other than the multiple-category-selection-widget (again, its broken). Some of the other advanced category or search plugins say they’ll do something like this, but I’ve not found anything that’s quite right.

Thanks!