3rd Brake Light flasher module install

Just bought a brake light flasher module (Stop-Alert) on Amazon to get my rear 3rd brake light to flash or pulse when I apply the brakes. Anyone have an install video link to help me with the install? Is there a place to splice into the brake light wiring harness that doesn't require taking down the headliner?

 

Sorry if this is a stupid question, but this a pix from the fuse box in the engine compartment or under the dash? And where is the module itself?

 

The cab fuses are on the under side of the module.

 

I can't find the tech info right now, but off the top of my head I think they get installed under the plastic door sill trim on the rear driver-side door.

I have one of these as a dealer installed option (truck was a dealer to dealer trade and the sending dealer installs it on all their inventory during PDI, apparently). I didn't want it and got my dealer to credit back the price but did a little research to understand where it is in the event it craps out and causes the 3rd brake light to stop working but decided to leave it as is so I didn't have to fix any wiring.