Custom connector I can't find anywhere

Beginner here.

I find myself in need of a connector I can't find anywhere online and I am struggling to make myself.

I have a Pioneer AVH-Z7200DAB, that I'm trying to connect to my 2004 Land Rover Discovery 2. The main iso-connector seems to be double-amped, once by the head unit, once again by the car.

I've found online that people have connected the 4-way female RCA pre-outs from the head unit to the 8-way male full ISO connector in the car.

I've bought an ISO extension, which I've cut into, and 4 x male RCA headers with positive and negative bare wires that I'm now trying, and failing, to crimp to the female side of the ISO-B connector.

I can't find any pre-made solution to this online, and trying to DIY this myself is making me want to drive as fast as the Land Rover can take me off a pier.

Does anyone know the name of what I'm looking for so I can google it better?