Well an opening would help more air to pass through, but even though its not vented, it's also not airtight, so the waves will still work their way through. The biggest issue you may have is rattly panels that vibrate against one another with a lot of low frequency energy back there. Hopefully you don't run into that, if you do, I recommend a little bit of dynamat.
Get a box for the sub though, don't mount it into a shelf like a rear deck speaker is mounted in most cars. The sub will need an enclosure to resonate in and truly produce any good sound.
hifisoundconnection.com and sonicelectronix.com both have a good selection of any car audio you may need, and at very reasonable prices, I've fared well with both! Sent from AutoGuide.com Free App