If you dont really care about APR then you can apply for joint application at dealership. With 45% down and 40K/year income is good to go. 99% you will get approved but APR can be around 16.99 to 22.99
Since you are talking about next March, use that time to increase your score.
I think having your SO on the loan will kill your chances of actually getting the loan at all so don't count on using her for your co-signer. Especially since the auto enhanced credit score for her will show much less than her current score (since she pays late all the time).
Put down as much money as you can until they say yes. If $11k is not enough of a down payment, then put down more. I too believe your issue is one of income and what you are going to have to do is have enough in it so that the lender feels protected in case of default, $11k down is a great start.
Do as suggested above: go to a CU. Do you have a banking relationship? You might try there too, but a CU is your best shot. Don't make any applications until you have money in hand (in your bank account).
Thanks for all the replys! This is exactly what I plan on doing. Plus I'm going to try and save until then, hopefully another 2-3k.
And Yes, I do have a banking relationship. That's also who I got my Secured card with. So I should have a pretty good chance of being approved.