Yeah its the sierra, my mrs is borrowing the car at the moment, I have a pioneer head unit (was already installed in the car when i bought it) which has 4 x 50W outputs. (Cant remember exact model, but i'm thinking of buying a new head unit anyway)
There is no rear speakers attached at the moment (hasn't been since i bought the car) and the new speakers replaced a couple of 6" x 4" speakers which were cable tied under the dash. the new speakers look very similar to these
http://www.polkaudio.com/caraudio/mm/index.php?s=mm651
except i'm pretty sure they are 4" x 4" speakers (they fit in the original speaker holes). I bought these ones as they are "Marine Certified" and as darwin is such a humid place for two thirds of the year, normal speakers seem to break much quicker.