Maybe something like this? Plug in AUX jack and it streams wirelessly?
TuneLink Auto hardware accessory for iPhone, iPod Touch & iPad
There are new HU's that are NAV ready, with DVD, IPOD, DNLA (bluetooth streaming)etc... For about $300-400.
I know you said that you don't want a HU. But I have spent a good deal of money on music audio cords, wireless streaming adapters for the ipod, radio freq channels that you have to tap into.. at the end of the day, some work, some dont. Some don't work in the hills, etc.
Spend the $200 more and get a decent HU that will look nice and it will also give your speakers more amplification and staging to sound better.
My $.02