Those ebay cables, if you open them (the scart area), will be plugging cables into the right spots but will be missing the resistors on red, green and blue.
Thank you, yeah, that's what I thought. I actually found that diagram earlier and opened the scart to see there was nothing in there. The seller advertised it as "brand new", but nowhere it said it was just homemade. I assumed it came from a factory, not a guy soldering in his basement (pretty obvious when you look at it). I could have done a better job honestly. I'll just ask for a refund as, on top of that, it has some heavy humming due to terrible audio grounding.