A DI-box would do the job, for example most of these would. Look in the specs if it supports speaker output (usually the maximum wattage is specified).
Note that most of them are mono, so look for a stereo version or hook up two of them parallel.
Also look at the voltage (if it is an active one), you need 12V for most cars.
Most DI-box's have balanced output, so you would either have to connect the cold tip to GND or leave it unconnected (the manual of the specific DI-box should tell which is best).
Offcourse you would need a bluetooth interface behind it to connect to your bluetooth speakers.