$1 car chargers & bluetooth speakers

The answer is no. You can't power a bluetooth speaker from a $1 car charger. The $1 car chargers are only useful for charging because they make too much ripple. Only a very large filter capacitor can reduce the ripple enough, in which case a battery is smaller. A giant filter capacitor is still more convenient than a battery. After years of taking 3 batteries out of the vehicle for charging, it's so convenient to only have to worry about 2.

The car chargers can be tested with an 8ohm load. The ripple is affected by output capacitance & battery impedance. Newer batteries can power bluetooth speakers more easily. The Rudung creates 100mV of ripple. The 10000uF output capacitor + 1R resistor reduces it to 50mV. The 10000uF with a new battery produces no ripple. An inductor on the output makes no difference.