Without seeing a pic, I'd guess that you have the "HiFi" stereo option that was offered from 2007-2009. It was replaced by the Harman Kardon system in 2010.
http://www.motoringfile.com/2009/07/21/new-hk-option-to-replace-hifi-stereo/
It has 10 speakers. The rear speakers are coaxial (separate tweeter and woofer speakers packaged in a single unit). Technically there are a total of 4 rear speakers.
