... disappearing down behind passenger lights - and re-appearing at drivers side loom ...
That would NOT be the dreaded "under the Xantia radiator you will find a rotten cable loom" syndrome - would it ?
Anyways - if its a +ve directly fed from the battery - I see absolutely no junk party in feeding your newly found feed point directly from the battery - with a new wire the same gauge as the one diverting out from the battery fork ...
Clearly its a bad +ve feed for the fuse/junction box - or ignition switch - you have found.
Xantia totally dead ignition - Alarm immobiliser!?
Moderator: RichardW