Technical Fiat 500 - Central Locking/Locked Boot

[Piccolo Nero e Bella]

Funny thing about it being well known though - FIAT still won't fix it for free. And they should.

And I still can't find the manual latch, unfortunately.

Unless the US model is substantially different it is next to the boot latch. Lower the seat back to gain access to the boot and you should be able to see it (and maybe even reach it!).