Most 300c's have amplified systems. Which means you will need to go to the amp to pull a good signal with enough voltage. I would recommend hi-lo converter and a line driver to increase the voltage going through your interconnects. You will also need to find out the impedance of your oem mids and highs if you plan on changing them. If the factory amp wants to see 1-2 ohm (some factory amplified speakers utilize this impedance to yield more power out of the oem amp) and you replace with a 4 ohm aftermarket speaker, your sound will suffer. Also, some oem amplified systems have the amp built on to the speaker...causing more problems when you want to change the mids and highs and keep the rest of the oem system unchanged. I think your car has a Boston Acoustics amplified system because of the nav, which you can add a sub amp and subs easily. But as far as replacing the highs and mids you may be stuck with the oem stuff becuase you don't want to change your oem head.