There is a separate mounting kit that is sold for the ST. The Westin bar is a universal/generic bar, you need to have the specific mounting brackets for the ST. The Westin bar does not come with the mounting brackets if bought at a general parts store such as Pep Boys and you will need to buy the brackets along with the bar. If you order it from e-bay, most of the sellers will sell the bar along with the bracket for the ST. Just make sure to ask the seller from e-bay if it comes with the ST specific brackets or if you're going to have to get the brackets/mounting kit some where else.
edit: if you buy the bar at Pep Boys, the mounting kit is sold for about $45 and it comes in a small brown box. Can't recall the # for them, but check out the Westin Product magazine on the shelf for the application # for the correct mounting kit for the ST