I was planning on using a slave tape player from an old 90s GM car, with the data cable between the stock unit and cassette player. Then the 3 wires for audio (l, r, ground) splice to a 1/8 jack. Looks like yours uses a similar idea. From what Ive read, hit the play button twice to access the AUX input. Also, what is the other end of that harness (circled in green) look like? Is it just a 1/8 jack?