Is the Tundra Exterior all metal?

No one said it would be new or that Toyota has never done it on a vehicle. It's not plastic on the 3rd gen Tundra, so what is/was done on the 4runner doesn't matter.

 

What are you going to do once you know the material? You going to whip together a FEM model of the bed and input the modulus of elasticity, yield and ultimate strengths for the materials and make a pretty rainbow picture of the bed deforming under your load conditions???? Thus what loads are you trying resist here bud? What are you going to do with the material information once you know what it is?

Put a hole in the bed and put something behind it to spread out the load like a plate washer and guess you'll just have to see if it holds. If your going to rely on the bed for say a lag bolt or self tapper to not rip out then thats probably not going to hold for long. Then again that probably depends if you're trying to secure a fleshlight or a 100lb floor jack to the side of the bed. You work in aerospace throw some ideas out there for different types of composite resin material...

What "fasteners" does your Aerospace brain have floating around that you think would work for a composite bed?

 

I was originally thinking the bed was more plastic than fiber filled, but it seemed pretty stiff to be just plastic alone (sorta felt like Delrin when I first touched it,) and it "sounded" like there might have been a metal layer (which Delrin sometimes does depending on its shape,) which apparently isn't the case. So self tapping and wood screws are out. Hole quality is the first issue to contend with, and drilling through many composites leaves a crap hole using conventional bits. There are dedicated drill bits ($$$$) for the CFRP types I'm most familiar with that leave a very clean hole, unlike a conventional drill bit that shreds the fibers all around the hole, weakening the material around the hole. It's pretty dramatic, and I've come to appreciate the problem. Diamond coated drill bits are likely the less expensive option, but "expensive" is sort of relative here.

As far as fasteners, the norm in aviation for composites is tight fitting fasteners that aren't driven in, but expand in the hole after placement. Again, very, very esoteric stuff. Either way, tight holes and keep the threads out of the hole, as they tear up the hole all by themselves. Rivet nuts come to mind as reasonable options, if that helps. The bonus is that it keeps you from having to get to the back of the hole.

While googling for options, this came up, and it relies on some localized softening of the resin as the fastener is installed. These methods require a thermoplastic composite (it melts with heat,) but will not work for thermoset (epoxy types, which based on on the descriptions provided, this bed seems to be.)
https://www.semblex.com/en/technologies/additional-fasteners/flowpoint-delta-pt/

Given that my experience is mostly with aviation, and composite fastening in this context, that's my worldview (my little box) when it comes to this sort of thing, and my familiarity with other industries is obviously more limited. There might be other things out there that I'm not aware of.

I did notice as I looked at the fasteners Toyota used and how they attach them to the bed, they don't have fasteners driven into the composite; they sandwich the bed between the bolt head (or rail) and a metal backing structure, so that was a huge clue for me. That method could have been for production facility, for hole integrity, or both. Hard for me to say. Other snide comments are welcome.

 

Snide comments incoming.
From a non-Engineer standpoint, you're over thinking this, a lot.
It's fiber reinforced plastic. It's not an aerospace grade material that requires special drill bits or hardware. It's not that special.
If you want to get fancy with it, have at it I guess, but why?

There are areas that have an aluminum sub-structure, like the rear vertical walls by the tailgate and turn signal, but most is just the plastic composite.
The wall section is fairly thick, but I would not rely on any type of self-tapping fastener to hold any significant amount of pull out force by itself.

Take a plain old HSS drill bit, and drill a hole, or several. No issues with hole quality.
Make sure you know what's behind before you drill. Yes, you'll have to get to the backside and back up the hole(s) with a washer or plate.
Riv-nuts might be a viable option if you absolutely can't get to the backside of the hole. Just make sure you use enough to support whatever load you're applying to them.

 

OK, I know that was probably a typo, but there's no way I'm letting that slide without at least giving you some grief over it.