Upgrade speakers?

I have about $100 to spend to upgrade my 2010 JBL Nav system. I'm not a real audiophile, but I can tell that the OEM sound is a bit fuzzy/muffled, and I'm hoping to get at least a bit better than what I have now. (I know it's trivial compared to what some of you guys have spent, but it's all I have to work with for a while). Never having upgraded a stereo system before, I'm assuming the logical upgrade target would be the speakers. I'm wondering if anyone can help me with a few questions:

1) Would it be better to put 3-way coaxial speakers in the doors and the dash, or are there component speakers available in my price range that would produce better sound?
2) Would I be better off spending the money on the two front door speakers, the two dash speakers, or upgrading all four? (for various reasons, upgrading the rear door speakers isn't feasible this year)