Well, I did take apart the whole lower part of the dash, so ...
I am just trying to figure out the sipliest way you know. I don't believe there is an easy way to ground where the HU is, you'd have to drill a hole for that. Since I don't know what is there and what is not without the airbag, take out the glove compartment and see if you see any piece of bare metal, if not, you can go further, take out the OBC and ground it there to one of the vertical posts. You would have to drill a hole and I think you would have to take some more pieces off to lay the wire but it sure would be a good ground.