@DickB was referring to the step down converter you listed. It is a 12 watt model. It's undersized to power an 18 watt light.
XWST DC/DC Converter Regulator 36V 48V Step Down to 12V 1A 12W Reducer Buck Power Converter Waterproof Module Transformer for Golf Cart Club Car https://amzn.to/3FUDF2G
XWST DC/DC Converter Regulator 36V 48V Step Down to 12V 1A 12W Reducer Buck Power Converter Waterproof Module Transformer for Golf Cart Club Car https://amzn.to/3FUDF2G