I installed this:
Robot Check
I strapped it up in the back of the cart and ran power wires to it inside the same channel where the wires for the back lights run. I installed a 12 volt switch that I purchased at the auto supply store up front which allows me to easily and quickly turn it off if I need to. The switch also ensures the battery does not drain when not in use. It is a bluetooth device so I can listen to anything from radio stations to google play music, to pandora, etc. via the phone. The unit is water proof, runs on 12 volts, and is designed for bouncy enviornments.