How can I improve the antenna of my FM modulator? Answered

I have a car (2003 Mitsubishi Galant) with the radio antenna embedded in the rear window.  I want to add an antenna to my FM modulator to improve the signal quality.  How can I determine the best antenna length and placement?

I already have information on how to mod the device. It's a Soundfeeder SF100.

And yes, this will become an Instructable eventually.  :-)