I would dump the stock E450 stereo and speakers, they are so poor sounding. Ours stopped working, so I replaced it with a Pioneer 2-Din DVD unit and 2 Infinity Kappa speakers. This unit also has a aux input that has a mini-stereo plug cable attached that we can plug into our smartphones. There is also a USB cable brought out that can have a USB memory stick loaded with music attached.
This produces great sound while driving. While stopped we primarily use the 5.1 surround sound DVD system that came with the rig.