what's this wire?

[oldnick]

in the course of changing the starter battery yesterday [ hint , although it is just possible to do this without removing the trim don't bother , we wasted more than half an hour getting the old one out , removed the trim to get the new one in , much quicker ] discovered a thin black wire not connected to anything near the earth pole

after much hunting about found where it must go .......the battery cover has a proper earthing point to connect it to

but as the cover is plastic WTF could that do ?
what am I missing here?