I found a nice website with pictures about how to replace a broken subwoofer on my late 2008 Macbook. In the following tutorial the subwoofer is connected to the speaker, but I only bought the subwoofer, and I don't even want to replace the speaker as that works very well. I feel like I want a confirmation that it will be possible to replace it with no problems or any tips.
http://www.ifixit.com/Guide/MacBook+Unibody+Model+A1278+Subwoofer+&+Right+Speaker+Replacement/766
http://www.ifixit.com/Guide/MacBook+Unibody+Model+A1278+Subwoofer+&+Right+Speaker+Replacement/766