Don't take this the wrong way, but your looking to get a car that almost 100% of your annual income. You really shouldn't spend more then 20% of your net monthly income on a car. If your a follower of Dave Ramsey he and others believe it shouldn't be more then 50% of annual income meaning if you make 50k a year the max car your looking at is 25k. Just my 2cents, but shady dealers will approve almost anyone just expect to be paying 20% interest.
There is a reason you were only approved for 14K and 15K, they think that is about the max financial load you can handle. Going beyond that is probably overextending yourself. Maybe not now, but a year down the line you may have way more bills for whatever reason....
You say you have a trade-in worth 3-5K? I assume it is still drivable then? I would suggest to keep this one for one more year, and set aside the payment you would be making if you would get the 20K financing. 2 advantages: you can see if you can handle the financial burden of that kind of loan, and after a year you will have a significant downpayment to make on your next car.
Anyway, just my 2cts.
I tried roadloans and got approved for 19k at 11%apr for 72 months.
Yuck, what was the rate your CU offered?
With your downpayment and trade you would be up around the 20k mark with the CU loan. Plenty of great vehicles to be had for $20k.