It will turm on if I adjust the dial by the wheel but whatever mechanism makes it turn on when the door is open does not work. Is it a fuse or a switch or a relay or something that operates this?

 

What Year Ranger?

Does light come one when passenger door opens?

Does it have Keyless entry?

 

07, yes on passenger and no on keyless.

 

Then the drivers door switch is broken, it is inside the door by/on the latch mechanism

Door switch has 2 wires, one is a ground all the time, the other runs to the SJB(smart junction box) which is the Body Computer, replaced GEM in 2004
When door is OPEN switch will CLOSE which connects that wire at SJB to Ground, SJB then activates Dome light
This switch also activates "DOOR AJAR" light on the dash, so if it doesn't come on when drivers door is open then that would confirm switch issue verses relay or fuse issue

Google: ford door ajar switch replacement

Its not all that hard to change, so buying a 3rd party switch for 1/2 the price or less, isn't a bad choice in my opinion, if it breaks it won't strand you on the side of the road like a 3rd party fuel pump might

 

I opened the door and started pushing in the little latch switch and when I push it down the light comes and the door ajar ding does too. As soon as I let go it stops though. Looks like I'll have to take the door apart and clean the latch to get it working again.

 

Some have said WD40 or similar spray lubes can get it working again