I had to get at that same area when I replaced my rear struts. One of the bolts sheared off, so I had to drill it out and put a new bolt in its place to hold the strut.
There IS a way to reach the top, but you're not going to like it. It's a huge pain in the ass, because that area was not meant to require maintenance.
You need to (in this order):
- take out rear bench seat
- take out rear seatbacks
- take off the 2 rear side C-pillar covers (be extra careful if you have the side-impact airbags)
- take out the rear deck cover
- take out the seatbelt mechanism on the side you want to reach
Then, where the rear seatbelt is anchored, there's a gap that you can barely stick your hand into to feel to the top of the strut mount.
It's a huge pain to get at, but all that disassembly only takes maybe an hour - even less after you've done it a couple times. The worst part is how awkward it is to actually do anything even after you've got access.
EDIT: if the threads spin at all, it means it's been replaced at some point. The original bolt is actually welded to the frame (the 12mm one). The only way it would spin is if the original was drilled out and a normal bolt (unwelded) was used to replace it, like what I had to do.