Look to see if you have a 12v light fixture nearby. If so you may be able to run the wires by simply and gently pulling down on the ceiling and pulling the wires through.
Any reason not to remove the existing fan from the bathroom and replace with the Fantastic? I took one of the power vents out of our Cruiser and replaced with a MaxxFan. Although I have two Maxxfans, running just one creates quite a breeze. Running one in and the other on exhaust creates a hurricane.