Hi @jassin000 and @Lemony Vengeance.@Crimson Kino if your Diamond is complete working for 1player all you need to do is add a K harness (thats what Taito calls it).
I believe @Lemony Vengeance still makes these for a reasonable price.
You CAN order directly from Taito, but they won't sell you just the K.
You'll be forced to buy the entire harness (J+K but you already have J) and its like 180$ shipped with fees.
Full J+K harness from Taito
Me making a homebrew K only (missing crimp/connector)
The receiving harness comes with the cab and should already be inside yours (if 1player is complete/working).
What do I do if my receiving control panel wiring harness doesn't have a "K" connector? I have a Diamond Orange (Magicians Dead). Even my P1 connector is different and I don't have a P2.