Capcom Auto loan questions?

I have a few questions about co-signing on an auto loan with capital communications federal credit union. i got approved for 10000 auto loan but i have not taken it out yet. my girlfriend, who is 17 wants to take one out from the same bank but she has no credit, and therefore would need a co-signer. i am 20. i was just wonder, if i co-singed for her, would i get approved for around the same that i already got approved for, and how does it work if she is unable to make payments on the loan she has out? also i was wondering if she payed off about half the loan, would her monthly payment change to reflect the payment so she would be paying for the same amount of time but pay less per month, or will it just make her pay the same per month but for a shorter amount of time? help me please! haha thank you very much.