Hi I'm still getting to know my XJL... I've been trying to figure out what this button is for.
please enlighten me.

 

That's the rear fog light switch.

 

It releases the horizontal jellyfish.

 

Was my guess. I had my nephew stand behind the car while I pushed the button, he told me he didn't notice any changes. So I was puzzled.

Thank you

 

It's illegal to turn that on in the USA. It's a Rear Fog light. Really bright red light. It would impaired the vision of the driver/s or people behind your car. It's only used in Europe while drving into thick and heavy fog

 

I turn that light on when driving in our Texas torrential downpours which are worse than London fog.

 

Don't plan on using it tbh.. was just curious.

I live in FL... so I can relate to the downpours

 

It is only enabled with the headlamps operating. Proper use of this feature will distinguish you from all the morons (in their silver cars and SUVs) driving in downpours or fog with all their lights Off.