The bolts holding on my rear license plate don't seem to want to come free. They just spin when I try to remove them. They are 10mm bolts. There is very clearly a nut of some kind on the inside that I can't get to and is why I can't remove the bolt.
I sprayed the bolts with PB Blaster to try to loosen them bit it didn't help. The vehicle is a 2007 Toyota Prius.
After advice posted to the comments of this question, I ended up looking at the backside of the bolts to size up the situation by pulling off the hatch paneling (how-to: https://www.youtube.com/watch?v=n3EJpG59rhw) and found this:
I tried to hold the "nut" with a pliers while turning the bolt but was unsuccessful. Instead, I used a Dremel tool and a cutting wheel to completely remove the bolt from the front:
Then I found a replacement kit from my local Napa auto parts store:
I had to slightly increase the existing hole diameter to fit the replacement mounts, but they fit snug and I'm not too concerned with them falling out anytime soon:
The finished mounted plate:
