Ok. Looks like I've posted to wrong forum, but anyway
If anyone cares, could you please move this thread to F-series hacks/mods?
I have disassembled my AV head unit, re-read the service manual and now it seems I know what's the issue.
F-840BT is missing 20-pin FFC cable from AV board to IF-PCB. This cable carries majority of audio and video signals, the only exception is the backup camera.
The fuse connecting the Preouts Ground to AV-units ground plane was also not soldered.
I'm going to buy the 20-pin cable and if everything goes well, will create a video documenting the modificat