T-mobile says the new Dashh has this:

Stereo Bluetooth® wireless technology      Connect your phone to your
computer or stereo headset without any wires.

Can this be used somehow to play mp3 via my cars stereo system? If so,
how?

My Volvo XC90 came with an after-market motorola handsfree bluetooth ..
But its very simple.. no indication I can play music audio through the
device. though it works great as hand free tool for phone conversations