The inside door handle attaches to the (Ford nomenclature) "door remote control and link assembly."

Attached to the remote control by a plastic clip...is a long rod that actuates the door latch.

The USUAL causes of handle failure:

The door remote control has a small coil spring inside that can break.

Or...the most common reason why the door won't open > > >

On each end of the rod, is that **&%^ plastic clip (there are 2) that hooks the rod to the remote control on one end, to the latch on the other. The little darlins' break apart from age.

I'll betcha it's one a those clips...available from any parts store, or FoMoCo.

EDIT: E6AZ-5421952-A .. Link Rod Clip ~ Use for door or tailgate rod / Ford list price: $1.11.