I've never figured out the point of that wire either, other than just being a tether so the panel doesn't escape. Its not like a piece of plastic needs an earth!
I'm also curious what trim you had to remove - do you mean the whole boot side carpet thing?
Once the cover panel is off it just lifts out of the hole on all the D2s I've worked on. Granted the battery terminals are a bit fiddly, but I don't see how pulling any trim would help with those?