Just had a pioneer DEH8700DAB installed and I didn't need any harness as one came with the unit so it was just plug and play. I did a lot of research and turned out I didn't need anything extra (well I needed a DAB antenna), luckily I was able to return it all. BTW it's a UK crossfire if that makes a difference.
Not sure if it does. I know other installs I've done, (granted its been a few years), required a harness. I would prefer the easier Metra, but want to retain the factory amp. I've been researching for days, but the only hits I get seem to redirect me to the forum.