Still working on the continuity checks,
but I found an old Sega Model 1 AV out (only RCA Mono and Video) plugged it into the Neo Geo CD Din port and - it works.
this connector only uses/has pins 1-5 on the Din. Not sure if that helps narrow down the issue or not.
I can plug in this AV cable, plus the yellow RCA and they both output fine.
When I plug in the Scart Cable and the yellow RCA, the scart is still "No Sync" on the ossc but the other Yellow RCA, becomes black and white and the sync is all messed up (scrolling)
Not sure wtf that means lol