[J_Speegle]

Under the carpet & underlayment, passenger side, where the passenger's feet would sit since the firewall panel travels down, then bends rearward to attach to the main floor panel about directly under the face of the dash panel. Can sometimes be found covered with a swipe of seam sealer if not visible to start with

 

[Bofa]

So only visible from inside the car?

 

[J_Speegle]

Yes since its located typically over where the frame rail passes