Just to confirm, under the dash you connected to the red/yellow wire correct? On my unit I had to connect the reverse gear indicator wire to the red/yellow wire under the steering column. In the video you mentioned he has it connected to an RCA cable. Even though I have an alpine unit, it...
One of the connectors, I believe the smaller square one is for the original tape deck (assuming you are coming from the stock radio). I just installed an alpine head unit and I didn’t use that connector. For the rest of your problem I can’t help as I bought the wiring harness pre configured from...