It seems like under the passenger seat would give you enough room. I would be worried about how well the remote contol will work. If it is IR based, you will probably have trouble during the day as your car is flooded with IR already from the sun. Also, if it is under the seat, you'd most likely be depending on reflected IR signals rather than direct line of sight.

What about the GUI? How do you plan to see and choose which songs or playlists you want to listen to or are you just using it blind?

Also, you might experience some problems when you try to play music after you're car has been basking in the summer sun all day. Depending on conditions, your car can reach over 140 inside. The disk enclosure combined with a 3.5" disk will build up a lot of heat. 3.5" drives draw much more juice and produce much more heat than 2.5" drives - so be careful.

Let us know how it turns out. Sounds very interesting...

-dan