And something failed inside the mechanism. Then the handle broke inside. How do I open a deadlocked door.

Interior panel is removed.

Deadlock motor is working but not disengaging

OK managed to unlock it

1 fight off the cover and remove it, horrible job
2 grab old tailgate lock and find deadlock mechanism
3 find it with fingers and try to push it
4 get screw driver in and push like mad
5 reconnect outside handle and open

Whole mechanism is very stiff.

New lock time

Inside handle was just unclipped

https://www.ebay.co.uk/itm/143227335761

Oh well

Worth a try