So I made the mistake of co singing for someone that isn’t making payments or even attempting to make payments. The car is registered to both of us so I was thinking what if I get them to pick me up and then when they get out, I jump in the driver seat and leave. Would that be illegal? I don’t know what else to do. They won’t give me the car back so I can make the payments myself and I’m sure not going to pay it while they keep it.
You certainly can't legally steal anything, ever. If it is legal, it is not stealing. In particular, if it is your property, it isn't stealing.
The question is really, "is it your property"? This will be jurisdiction dependant. I am familiar with England and Wales, and other common law jurisdictions may be similar.
The fact you co-signed for the car definitely does not make it your property. It just means that out of the goodness of your heart, you agreed to reduce the finance company's risk by promising to pay if your ex-friend didn't.
In E&W, I don't think it is possible to register a car jointly, but there is a long page explaining that the registered keeper is not necessarily the owner of the car.
My guess is that the car probably does not belong to you, so if you take it you will probably end up with a conviction for theft.