I'm presently trying to figure out what size bolt fits into what appears to be a sleeve-type nut that is welded to the interior of the trunk, but haven't had any luck yet. Any ideas?

The receiving end is pictures in the top right area.
I've been through:
m6-1.0
m7-1.0
m8-1.0
m8-1.25
1/4-20
1/4-28
5/16-18
5/16-24
It's smaller than the 5/16-24 & 5/16-18 & m8s
larger than the m6, m7, and 1/4s
Any ideas? I'd like to use the three that are in the ceiling in the trunk. Two just next to the speakers, and one in the center.
Thank you!

 

I've tried every bolt that the local hardware store carries, and had no luck. Could this be a proprietary bolt size? Does anyone know what might use these in a different model? More importantly, if they are utilized, would it be for something on the other side of the sheet metal ie. in the 'parcel tray' area by the 3rd brake light, or on the same side of the sheet metal, which would be within the trunk?
I'm curious because I'd like to mount my amp near the 'ceiling' of the trunk, and I'd rather not a) have those tack welds fail, or b) have my amp fall.
Thanks.

 

Those are there to anchor tether straps(*) for baby seats in the back seat, so ordinarily a bolt would be punched through the carpet/cardboard trim and installed from the top side inside the car. Have you tried threading from the top? The bottom of the threads may be intentionally distorted to work as a lock nut.

(*) which some baby seats require so they don't pitch forward in a crash.

 

No kidding! Interesting.
No, I haven't tried to thread any bolts from the top - the idea of pulling apart all of that trim to remove the carpeted portion on a gamble isn't too appealing. If I could find out what measurements applied to the bolt that would be used for those tether straps, I could just come in through the trunk and chase the threads with a tap. The dealer claimed ignorance, due to the year of my vehicle..after 10 minutes on hold.

According to the interweb, it should be part #82410-SE3-C01 for the plate assembly kit. Now if I knew what was included in it..

Hah! After trolling for about an hour, I was only able to find that it was an 8m bolt with no thread pitch information - thought that I'd check the FSM and there it is!
8x1.25
Thanks for the information and setting me down the path!