Many many moons ago when I was into this stuff, those adapters came with the headunit lol
NO they jolly well didn't! There is indeed an ISO Loom that comes with most stereos, AND they still do, but the configuration of the mating half in many Euro Cars wasn't compatible with the head unit. This was the case for VW Group, Fiat, PSA, Alfa, Vauxhall/Opel and possibly others - the plugs looked the same, but they would cause problems if directly connected to the head unit - things like resetting the milometer in a Peugeot to Flattening the Battery in VW Group or Vauxhal/Opel car. Even today, with the new Quadlock standard, different manufacturers wire the head unit plug differently - no-one will sit down and sort a wiring standard for head units as they don't want to make it easy for the aftermarket to take profit from them.
When I worked at Autoleads, many moons ago, the sale of these items made me a very nice living thank you very much! I chose a TVR Tasmin Wedge, a Rover 825SLD Lowered and Limo tinted and a Suzuki X90 as my modes of transport....... cos I'm pretty sure I'm a little mental.