Xj 600 random plug

Hi I’m looking to run a usb charger on the xj600 I have found a random 3 pin plug under the seat it has a switched live in it but I would like to know what it would be there for. 

I would add a photo but I can’t work out how to 

it is a inline 3 pin male plug that comes from the main loom with two other plugs that have been linked out

help would be nice but understand if no one can with the little information I have given