It will build your credit
What do you means by being second?
Did you co-sign for her? I cant see how and why they made it possible.
Surely, if your score isn’t good enough to get the loan even with your girl friend co-signing with you, than how on earth can you guaranty the loan in case that she doesn’t pay?
What ever the reason they signed you up like this, if your name is on the contract, and that dealership or the financial institution that gave you the loan reports to a credit reporting agency than it will show up on your credit report, and assuming that there will be no late payments it will help you build good credit and even possibly will raise your score.