What version of ASPDNSF are you running? You are posting in MultiStore, but mentioned ASCX template file - which suggests you are using a pre 9 version.
Would you like to change the template for all Topics, or only a select set?
There are a few ways you can achive template switching for topics. One way is to override the template from the page's code behind. In this instance, driver.aspx.cs (responsible for topic pages).
protected override string OverrideTemplate()
Please take caution to test on a development site first, before updating your production site.