Has anybody tried the Cree Q5 LED 7W back up light bulb being sold on Ebay? It looks good and the price is reasonable. I asked the seller if it needed a load resistor in order to work properly and he said no. The back up lights in my '06 GT are pretty wimpy.
They are item #310745046807 on Ebay and run $5.52 ea. free shipping.

 

Cree is a great company, they made the LEDs that I used to make my custom blinkers and are nice and bright. As far as if you need resistors, I couldn't tell you exactly but even if you did they're only a few bucks as well. Good luck!