mono fm radio

I want to build an fm radio into an old case and using the original speaker, which is quite good and a reasonable size. However, with just one speaker I need mono output from the fm radio module controlled by an Arduino nano. Any suggestions on how to achieve this?

The simplest way is just to combine the two signals through a resistor on each line.

It is best to do this on the input to the audio amplifier rather than the output.

Alternatively there may be a way of just telling the module you want mono.