[oredith]

I've never done the rear trunk thing, so take this as you will. If you're planning on mounting it under the trunk like the tech article, i would think the airflow will be enough to disapate most of the heat, but having a safety is always nice. If you're thinking of mounting it IN the rear trunk like Mike Z., I would think a fan would be required unless you plan to do some custom ducting to guide air through the cooler. the underside of the trunk isn't exactly a high flow area, and will be subject to the rising heat generated by the tranny (ever put your hand on the trunk pan after driving?). another problem is that if there's no outlet (besides the mounting hole) for the air going in the trunk, the most you can expect is radiant cooling from the stagnant air.
good luck, i personally would add the fan just for the peace of mind..

Jeff