Hyundai i800 - Parasitic drain

[Antoin]

Completely out of my comfort zone with car electrics but I said I would give a mate a hand with his Hyundai i800 minibus.

Drains the battery every once in a while.
So obviously something is draining it overnight.

This morning he came out and the rear lights were on, sidelights and brake lights. Then he drove it and it went back to normal ie none on when they shouldn't be.

Thoughts on what would cause it?
Ideas to diagnose it?

Have a multimeter and that's the height of it.

Thanks
Anton

 

[NI_Volvo_Nut]

Pretty sure those are CANBUS lights so I'd look for water at the control modules.