The base 6 has 2 ohm front door and 8 ohm dash speakers wired in parallel. This means the doors receive roughly 4 times the power as the dash. In addition, the dash speakers have a very aggressive filter capacitor as well.
Many users have replaced the dash speakers with Kicker, Infinity, or Faital 3.5's and liked the results (search the forum for those words for details), although some mention that using 4 ohm dash speakers causes them to be too bright/loud (the power balance shifts with the load change). Some folks also recommend some "bass blockers" (filter capacitor) for those dash speakers. I would also caution that lowering the overall impedance can put the head unit's internal amplifier at risk of overheating or failure.
There are a number of threads on this; a lot of folks have done this upgrade - hopefully this gets you started in the right direction!
