Wow, this was a brain-buster -- I couldn't find the correct pinout for the 3.5mm plug on the DJI downlink transmitter *anywhere*. However, I remembered that the AVL 5.8 Downlink was originally designed for the Zenmuse Z15. That gimbal has a GCU with a 3.5mm jack so you can plug the TX directly into the video out from the gimbal. (The newer Zenmuse for Phantom uses the same pinout so is also compatible with the Downlink TX.) So wire your 3.5mm jack to the iOSD this way:
"Tip" contact = 12V+ power [you don't need this, leave it unconnected]
"Ring" middle contact = Video signal, wire to iOSD Mini orange VIDEO OUT
"Sleeve" bottom contact = GND, wire to iOSD Mini brown Ground (outside wire next to VIDEO OUT)
That should do it!