open-air

I'm looking to upgrade the complete audio system of my car. I need a new (1) stereo, (2) speakers, (3) amplifier, and a (4) subwoofer. Note: price for parts isn't an issue, the cost for labor (for things that I can't do myself and keep in mind i'm a novice when it comes to car audio) will still...

So I'd like to add a subwoofer to my car (1995 BMW 318i Convertible), but I don't want to just stick a box in my trunk. I've seen posts about IB/open-air setups by mounting a square woofer to the ski pass which seems interesting, but I'm not to good with this stuff and I'm not sure where to...