[blakehaas]

Rear back wheel well driver side closest to bumper.

The black nut or fastener fell off. Not sure what it is to get another one.

 

[Fauropitotto]

Mazda part# UH71-68-615

It's about $1.33 for OEM.

It's called a "Splash Shield Nut" or "Wheelhouse Liner Nut", and you can find it under the quarter panel component sections on any Mazda parts online store.

 

[blakehaas]

Thanks!