This is on behalf of my fiance.
Your Age: 22
How long have your been on file with the CRA’s?
3 Years

Credit Scores:
Equifax? 590ish
Experian? 590ish
TransUnion? 600ish

Your Credit:
Rate your OVER ALL credit from 1-10 (10=BEST) - 5
Rate your installment Credit from 1-10: - 7
Rate your revolving Credit from 1-10: - 6

Public Records?
If yes, please describe:
None
Auto Loans:
Do you have an open auto loan? Yes, co-signed with sister
How many auto loans are presently open? 1
Are you trading in your open auto loan? Yes
Are there late pays on your CURRENT auto loan? No
If yes, please list the time frame.
Current open auto loan is financed with? America Honda Finance Corporation
Current open auto loan payment? 359.61
Estimated amount you may be upside down in this vehicle? 5000ish
Rate this open auto loan from 1-10.

Credit Cards:
Total number of revolving accounts: 0
Percent utilization overall: Unknown

Your Info:
Total Provable MONTHLY GROSS income: $2500
Provable via computerized paystubs or what method: Pay stubs

New or Used Vehicle:
Please describe desired vehicle: Mazda6
roughlu 21,000 MSRP

Please leave any comments that your feel may be pertinent and may lend added weight for a favorable outcome.

This is a co-signed loan that is 2.5 years into. Co-signer is upside down in the loan. 10k own, trade in value 5k.

I don't think the main problem will be with procuring financing, but having a lender finance the amoutn you need to cover the negative equity, unless you make a cash investment or choose a new vehicle that will have a cash rebate to offset a portion of the amount

Also she will be putting down 4500 for down payment.

Then there should be no major obstacle. You are basically covering most of the negative equity. Just a small portion of that to be added to the new loan plus the taxes and fees should be fine.