It turns out this car has a wiring system not used by any other manufacturer in the known universe

But, of course! We are, after all, Peugeot!

It's pretty much impossible to get hands-on experience with late-model Peugeots here in the U.S., so I can offer only a general suggestion....

Perhaps the best $30 I spent on my new car was for the electrical shop manual. thankfully Subaru sold it separately. It was invaluable in performing some alarm system and fog light modifications without guesswork. If Peugeot offers the equivalent, it should show you wire color codes and connector positions for all of the stereo harness/connectors. You still have the challenge of finding connectors to avoid cutting wires in your leased car, but at least you'd have something to go on if you have to rewire a connector yourself. At least some of the connectors are ISO.

I'm guessing you might have meant "line-level" outputs to the JBL amp?

I will now butt out and hope that somebody can offer you more useful information!