This post won't interest regular readers, but I wanted people in my boat to find it.
The Monster Cable FM transmitter for iPhones sucks ass like few products I've ever owned. As soon as I got it, I was looking to get rid of it. Opportunity knocked last week.
When I got the new, redesigned Mophie battery pack for my iPhone 3G, the first thing I noticed was that the port on the bottom was USB instead of the old iPod port. One quick test later, and voila, yes, now data (in addition to power) could be passed through the Mophie. This wasn't possible before. Soon I was dreaming about plugging my phone/Mophie combo into my car stereo.
But precious little information was available on the Web, and what I found said it couldn't be done. So I grabbed a USB cable and went to electronics stores. Pairing with most of the car stereos with front USB ports usually had one of two problems: the iPhone didn't recognize the stereo, so I was bonked with the annoying "This accessory isn't compatible" message, or the stereo recognized the songs on the iPhone but wouldn't allow me much control of them.
And then I found the Sony CDX-GT630. So seldom in life do you think "Wouldn't it be nice if such-and-such were possible" and it's actually possible.
It works perfectly, right through the Mophie. I listen to tunes and sort by artist, playlist, album etc. by using the stereo's interface. It charges the Mophie, which in turn charges the phone. When I get a call, the music fades out automatically. It's very nearly perfect. The only cost I've found is that, in that the iPod part of the phone transfers all control to the stereo (and its UI disappears on the phone), I can't watch videos while listening to the audio over the stereo. And I bet even this is possible and I just haven't figured it out. Either way, all I would have to do is use an audio cable. A small price to pay.
iPhone 3G with Mophie battery pack, velcroed to my dash and charging by USB cable from Sony CDX-GT630, which is playing song from the iPhone