Rear mud guard thingy

The one you're showing (2 pics of same??) is missing fastener? That would make it more vulnerable, prone to tear off. Aftermarhrt license plate bolts are a good sub, and not rustprone.

Our 2010 with 17" rims came with full liners (some sorta fibrous material) in the rear wheel wells. Without these the bracket looks odd, exposed and incomplete.

I think the term for the plastic piece is "spat".

The look with liners:



Kinda hard to see, but this is where I'm using nylon license plate bolts:



The original bolts were rusted doodoo.

 

Pics from another thread:

Splash guards | PriusChat

 

As far as I know the fibrous liner was standard on only level "V", ie: the US model with 17" wheels. Our Canadian Touring is similar. I'm not sure on the years, would speculate that'd be the case for 2010/2011, then in 2012 there was a minor makeover. I've got a vague recollection they started putting the fibrous liner on all levels, at some point, not sure.

The liner has a bunch of small diameter holes, that match up with threaded studs in the wheel well, and there's plastic nuts with an internal steel bit that screw onto the studs, holding the liner against the interior of the wheel well.

At the trailing corner of the wheel well theres a black plastic piece has some push-in fasteners that also capture that fibrous liner (they're hard to remove, I did it once. You need to release some internal clips, through the hole in centre. Think they're for screwing in rear mud flap option.), plus one of the plastic nuts with internal steel bit. Pics of those, in order:


The nut in above is the same one used at numerous locations on the threaded studs. You'd want to check first those threaded studs are present, I'd suspect they are; that they've just opted to omit the liners on all level except V.

The major stumbling block if you want to get the liners seems to be finding a parts site with a decent interface. I tried a bit just now, without success. Hopefully too the parts ARE available. Simplest might just be a visit to a Toyota parts department, see what they say.

Earlier thread discussing the liner:

Should the rear wheel wells look "unfinished"? | PriusChat