A cosigner or coowner cannot repossess a vehicle. That is something the leinholder does.
The cosigner of the loan owns 1/2 of the property if they are on the title.
Property does not have an income tax return.
That is an issue between the mortgage company, the buyer and the cosigner. The seller's only worry is selling the property and getting paid.
Many people cosign a loan for property they don't own. Many are uninformed of the consequences of cosigning. They don't realize they are agreeing to be completely responsible for a loan for property that belongs to someone else. If the primary borrower defaults on the loan and the cosigner must make the payments, the cosigner has no automatic right to the property.
The primary and cosigner on a car note are equal owners. Neither has the "right of ownership" over the other. This is a common misconception. Both may not benefit from the transaction, but both will be negatively affected if the note is not paid.
If you are not an owner of the property, as a co-signer you are fully responsible for paying the mortgage. You have no other rights in the property.
No. The car is not your property, nor do you have legal authority to sieze property. It would be considered grand theft. As a co-signer, you agreed to be just as legally liable for the debt as the borrower. That's why it's a good idea to NEVER co-sign ANYTHING.