Those are the plug parts I used. I'm very close to first test. I was going to finish installing lumbar support in the driver's seat first. Should do that today. May actually test it all today -- if I get up the nerve.
That isn't a pass-through connector. That just replaces one half of the X9331 connector, leaving the other side disconnected. It isn't known yet whether that is OK in the HIFI system (which I have). Schatzy62 says it works fine in his MINI, but I think he started with the standard audio.
My original plan was to make a pass-through X9331 connector. I got one built where the male and female housings were back-to-back (instead of the normal belly-to-belly connection

). However, as I put them together, I lost one connection and had to tear it apart. So, I decided to try just the male half, leaving the HIFI amp unplugged. If I have to take it to the dealer for a software upgrade, I may have to unplug my amp and plug in the HIFI amp.
My Q 450.4 amp has RCA inputs so I used a
Kicker ZI44 (09ZI44) 13 ft. 4-Channel RCA Audio Interconnect Cable instead of the wires you are using. Only cost $30 and has RCA plugs attached. I just cut them off one end. The wires are rather delicate, but not much is needed for signal to the amp.
You just need the normal tools: soldering iron, needle nose pliers, etc.