Is line level Bluetooth audio possible with iPhone?

Solution 1:

You'll want something like the Griffin BlueTrip Aux as it gets powered from a DC car jack (a.k.a cigarette lighter recepticle) and receives bluetooth music and sends it out a stereo analog connector to your car's AUX in / line in headphone jack.

There are many similar products that are cheaper and larger and the audio quality on some is not as good as Griffin's product, but you can shop around since there are several similar product on the market.

The part you list needs a USB cable and getting that out of a lightning port phone would be far more product than a simple BT receiver.