If you really wanted to get serious with this and turn it into a project, you could always do what I'm planning to eventually do; build a mini-ITX system to go in your car. Then, you mount a 7" touchscreen on your dash, install Linux on the system, load it with MP3's, plug the soundcard line-out into your stereo line-in, and voila! Bob's your uncle.
This also opens you up to some nice extras, like cheap GPS (all you need is a GPS antenna and some open-source software) and on-dash DVD playback.
Just make sure the stereo can do that as some brands have the USB port just so you can plug in an iPod/iPhoneI'm going to do this. I just bought a new car, but I don't get to pick it up till today and I didn't know you could do that.
I've been semi-following those little beasties and I can't help but wonder why they're not using mini-USB ports, tho it wouldn't surprise me if the mini-USB cost more per piece than standard size
Just make sure the stereo can do that as some brands have the USB port just so you can plug in an iPod/iPhone
but you're an amazing lover
Or you could just by a mount for your smart phone and connect it to your stereo. It does everything you just mentioned here and won't get stolen since you can walk away with it in your pocket.
I like my stereo. Has a USB port built in so just dump music on a thumbdrive and plug it in