MFJ-5124I cable schematic

Pin 1 (Key) connects to the Tip of 3.5 mm Stereo Phone Plug.
Pin 2 (Start) connects to the Ring of 3.5 mm Stereo Phone Plug.
Pin 3 (+13.8V) connects to the Center Pin of Power Plug.
Pin 4 (Ground) connects to the Sleeves of both 3.5 mm Stereo Phone Plug and Power Plug.