What I'm wondering is if you have to arrest 2 people in a vehicle, the first one NORMALLY comes out and you can arrest him. But the problem for me is how to get the passenger out of the vehicle because they always stay inside. If I have backup and they're with me throughout the chase, they'll normally pull him out but how do you, as a player, do it?

I think the only way to do it is to go over and try to steal the car, and before you get in, turn and arrest that person.

I have not tested it, as I am writing this on my IPAD, but try pointing a gun at them.... When you point a weapon at the peds, they usually get scared and try to run. I bet this would get them to exit the vehicle so you could slap the cuffs on! :-)

No those don't work...

You should be able to go to that side of the vehicle and try to get in, F key by default, and that usually gets them out for me. Just make sure you are running up against the passenger side door.

What about, AI sitting in the back seats of a taxi? Any suggestions with that? I'm just wondering what others do!

Nothing, you don't have access to them. You can always try to call backup, target the person with a gun and then hit '3', but usually that ends up in a shooting, not an arrest. And you still don't get to remove them personally. You just have to hope the AI will do it for you.