My mom won't get out of the care to come in the house. It's dark and I don't know what to do?

This is a risk to her safety unfortunately. If it was her bedroom she wouldn't leave, so what. But the car is different. So:
1. Ensure she cannot start/move the car. Get a lawn chair and a soda, sit and wait 10 min as recommended earlier and see if that will reset her. After 2x of that, then
2. Call the non-emergency police line and advise them you are having a problem getting Mom to come in and see if they will send an officer out as an authority figure to convince her to go in the house. Even if they won't send anybody out, they will at least have it on record that she's acting irrational.
3. See if you can set the car alarm so that if she tries to get out it will set off the alarm and alert you.
4. Offer her a favorite treat, in the house.
5. If after all that, she still won't relent, I guess I'd call the fire department to see if they will have her assessed and possibly admitted for a 72 hour hold for mental issues. Those brawny folk will just lift her out of the car, onto a stretcher and off she goes.
I'm curious how this turned out. Sounds like she's going to be a real corker as she proceeds further into her dementia or whatever is going on with her. Good luck.

This is when I would call for help. IE: police OR time to move her to a facility. IF this happened something else more extreme may happen. Find a good facility that CAN take care of situations like the one you just had :)

I know you probably have your hands full right now - but a little more info would help. where does your mom want to go or are you getting back from somewhere? Is she giving you any reasons why she wants to go wherever? Is another daughter involved - is it feasible to get her involved if this goes on for a while - instead of a pretender?

When my mother does similar things, I just look out the window and check to make sure she's okay every few minutes. I don't know what I would do if it was dark. Tired had a good suggestion. I hope it will work.

Keep a close eye on her but take the keys and leave her be for about 10 min. Then approach her like nothing happened and say were here lets go inside or something like that. If she has dementia or memory issues sometimes 10 min is enough to reset the situation.