The layout of the product listing is actually defined in includes/classes/boxes.php
class productListingBox is the one that generates the table for product listings.
The actual table layout elements (the tr, td etc tags) are generated in the extended class tableBox, so look into that (yes, the mixing of code and html tags is THAT horrible).
If you don't want to alter the table layout, but just change the information in table cells, then you'll find that it's loaded in includes/modules/product_listing.php
P.S. The code that I looked at didn't have STS installed, so there is a slight chance that STS generates the table somewhere else...