This just further illustrates OP's point, of how very limited this market segment must be

The buyers of this product are people who:

  - spend enough time in their car to want streaming audio

  - spend money on phone and internet 

  - have a desire to improve their car to have streaming audio

  - but not enough to replace the car

  - and also not enough to replace the stereo

  - but definitely enough to pay money to add it

  - and are willing to remove the item they are paying for every time they leave the car to prevent its attractiveness to thieves