Can a private party co-signer repossess an automobile from the primary loan signer?

Co-signor repo?? NO, NO, and again NO. As co-signor, you ONLY guarantee that YOU will pay the loan. It gives you NO other rights to possession. IF, IF, IF you are listed on the TITLE as co-OWNER, you may take possession. ONLY if you are on the title. GOOD LUCK.