The interior light on my 22 reg titanium Vignale only has 2 switch positions. If it’s in the “on” position they stay on all of the time unless I press the lens as in reading light. In the “off” position they’re off all of the time, not surprisingly. However, I can never get them to come on when opening doors, neither does my boot light come on when opening the tailgate. The manual, in FordPass app, is not helpful. Am I missing something obvious?

Have you had the switch in the middle position, ie not pressed on or off.

Owners Handbook, see page 97

https://www.fordservicecontent.com/Ford_Content/Catalog/owner_information/CG3657en-202012-20201215133259.pdf

The switch is in the level position now but I can only get a light when I press the lens. Nothing happens when opening the door.

I’ve discovered today that when I power off, the lights come on until I open the door when they quickly fade off. However I still can’t get them to come on when unlocking and opening the doors.

Battery might be low. It’s one of the things turned off to save power, as shown in the manual screenshot in unofix’s post. 

We have the same problem on a 23 plate Titanium X Fiesta. Just another irritation of the 'low' battery scenario, along with door handle sensors not working and AC. That and missing features compared to a 16 plate Titanium X are why we're getting rid of it. Very disappointed with it compared to the other 7 Fiestas we've had.

Yes. I would say it's low battery also. Does it in mine all the time. Lots of stop start traffic will charge it back up again. That's what I find anyway.