Whilst we love nothing more than a gloriously pimped our rickshaw, we do have to be a bit careful with our Sri Lankan fleet as they are all borrowed from local rickshaw drivers. This means that you're not allowed to do anything permanent. Any modifications you make must be able to be removed without leaving a trace.
Instead of thinking to wire in a stereo - which will inevitably go wrong, sound crap and drain your battery - we recommend taking a portable Bluetooth speaker. Not only does this mean you can take it to your room or to the beach on an evening, it also means you can take it home again with you afterwards.