I grounded it to the frame since I did not want to grind off any paint at the mount. The top of the mount is on the plastic bed rail cap, only the clamps are touching the metal underneath (painted). It might work but I didn't try it.

One tip, be careful removing the stake hole cover this time of year since it is getting cold. You may want to warm it slightly with a hair dryer since you have to flex it a good bit to get out it.