I co-signed for my now ex-husband a vehicle and he's now in jail.
He's since remarried. He gave me a handwritten letter giving me permission to take ownership of the vehicle but the letter was not noterized. Can I take possession of the vehicle since he's no longer able to make the monthly payments? What rights do I have as the co-signer?

He will need to sign the actual title over to you,
Or give you written POA notarized in a manner legal in your state ( some will need witnesses)

A handwritten note, with no notary will not allow you to do anything, if you are just a co-signer and not on the actual title.

If he is giving it to you, he wrote the letter, just go get it from where he has it parked