It's a common audio adapter from Radio Shack that works fine for the mono audio and video signal needed for the TurboVision. They also come in the form of a Y-Cable: Left and right female RCA inputs on one end, 1/8" TRS (tip/ring/sleeve, just look for two black bands) on the other. Plug your video connection into the red input, and don't worry, it doesn't hurt anything if you reverse the wires.
EDIT:
Thanks for the photos. Can you use the TV tuner for AV output as well as input?
The photos may look cool, but remember a separate controller is involved. No video output is available, otherwise why would you need a home console! I did see a photo of a tricked-out, modified GT once with video out and a controller input...
Anyone know where I can see it again?