Transit mk7 clarks mess conversion isolator for aux batterie

[Chungus]

Hi,
I recently bought a ex network rail mess van to convert into a camper.
A quick question I’m hoping someone can answer for me...
The isolator switch on the dash next to the hazard switch doesn’t work, I’ve traced the cables and it goes to a yellow plug in the fuse / split charge relay area in the welfare unit.
Does anyone know where or what this plug should be plugged in to or why it would of been removed? I can attach a pic of the plug.
Thanks.