It doesn't need to be a pulsed 5v, that's just the way it worked out with the old technology. Basically the contacts were closed 5/12 of the time (in a 12 volt system, 5/14 in a 14 volt system). That's what caused the pulsing. Here's a link to a Mopar site that has pretty good explanations:

http://www.allpar.com/history/mopar/electrical2.html

Even though it's Mopar, all the info applies to Mustangs as well.

I hope this helps someone.