Assuming you've never disassembled your Phantom or poked around the landing strut mounts too much, if you examine the white rubber grommets in the center hole of each of the four places where the landing gear is screwed to the frame, in one of them you should see a very tiny pair of wires, one gray and one red, just barely visible and slightly protruding. If you're lucky you'll be able to pull them out with tweezers. That is a 12V supply connected directly to the main board. If you don't find the wire protruding, Google how to open your shell and find the AUX power lead and push it out.
Then, take the power leads provided with your TX and either:
- Cut off the red JST connector from the end that does NOT connect to the video TX and solder to the wires you pulled out, red to red and black to gray.
- Solder a mating JST connector to the wire you pulled out, so you can plug the red connector on your VTX into that. This lets you plug and unplug power to the TX (desirable since it draws a lot of current and you won't always want it transmitting) without stressing the black Molex connector on the VTX board.
If you don't know how to solder and use heat shrink tubing to insulate the joint there are plenty of YouTube tutorials.
As for the video connection, without seeing the cable you purchased it's a bit difficult to guess, but generally the video signal is on yellow and the audio on red -- but that leaves you with no GND, which isn't possible. It would be unlikely for the red to be GND unless it's just a very poorly made cable, so I should ask: besides the yellow and red wires at the end, is there any sort of braided wire shielding around the yellow and red, just underneath the plastic jacket of the wire? If so, then there a good chance that braided shielding is the ground. Just expose enough of it so you can twist it into a third wire.
Your video TX according to the photo has a 5-pin Molex wire harness, so you'd connect yellow from the GoPro to yellow (middle) wire from the TX, and that braided shield (ground) to the black GND wire on the 5-wire cable. If that doesn't work, you can *try* connecting the red from the camera to the black GND, but I don't think that's going to be correct. If you can't get it sorted just post a pic of the GoPro cable and someone will figure it out.
