For those of you having issues using the dock AND the 3.5mm jack to send audio through to your car speakers, there's finally a music player that actually resolves this 100%.
Right now, it's only a "trial" version, which leads me to believe that it will eventually be a paid app, but the player is called PowerAMP! It was posted in the music player thread, but it's significant enough that I'm posting the download link from appbrain on here:
PowerAMP Music Player (Trial) - Android app on AppBrain
This Music player has an equalizer that allows you to control the volume level from the player! This solved my issue of the sound coming out distorted because the dock would disable the volume control!
Sorry for those of you that returned the dock because of that issue, but this player is definitely worth it. I'm still going to keep MusicMod because I like the form factor, but this player will be my dedicated music player while I'm in the car and my phone is docked.