I believe rear bumper droop on the HHR bodyshell (400/45 etc). is not unknown.
That said, there is a short video on the 'net which demonstrates a metal threaded rod device that clamps onto a seam behind the bumper sides, which is then adjusted to lift the bumper back to the correct height.
Has anyone knowledge or fitted a pair (one each side), or who markets them?
Regards to all,
Stan.