Just installed this today - a module that makes the Center High Mounted Stop Light (CHMSL) - i.e. 3rd Brake Light to flash on initial brake. I used the GS-100A, which you can find on E-Bay or Ali Express.
To install - remove the passenger side rear C-Pillar trim. Move the "Airbag" plastic cover to reveal a 10mm bolt. Remove the bolt,, then tug on the trim to free it from the clips holding it.
The black connector, under the silver antenna module, is the wiring for the CHMSL. The manual calls this a CHMSL filter. In the picture below, I have disconnected it.
You'll see two red wires coming from this connector. Remove the black protective covering, and trace the two wires back to see where they are going. You want to cut the wire that heads upwards towards the brake light.
Cut the wire. Connect the wire coming from the connector to the "in" of the module, and connect the other wire heading to the module to the "out." Connect the black "out" to ground - there a grounding point close by. The "in" ground does not have to be connected.
Tidy up the wires - I mounted the module under the antenna module with double sided tape.
Video in action: