actually i think you cant turn them on with just the parking lights. i believe they work independent of the main lights. and yes, to answer your question they can be operated independent of anything. there is a simple how to over at mustangworld, here is the link:
http://www.mustangworld.com/ourpics/fcar/fog2.htm. there are a few ways to do this, but this is the way they recommend and the way that i used. it works great and is easy.