The first thing you should do is contact your lender and find out the exact pay-off.

When you trade a vehicle in you are selling it to the dealer for a wholesale price. The dealer looks at your car then he figures what he needs to do to it before he can put it on his lot for sale. They also look at the fact that if it does not sell how much they can get for it if it goes to the auction.

Here is an example: You find a car you like say for $10,000 your trade in gets $17,000 and you owe the bank $21,000. So you are $4,000 in the hole. That $4000 gets’ rolled into your new loan. That would be an additional $80 worth of payment a month just on the roll over. Remember for every $1000 you finance that equates to about $20 worth of car payment.

You said you wanted to find a car for $5,000 your problem is getting a bank to loan you the money on such a small amount. The bank will book out the car which could be worth around $3,500 and then you are rolling over $4,000. So, unless you have great credit this deal has a good chance of not being bought.
Your best bet is to find something that is more expensive.
Rmember cash down is a big factor. If you have $3,000 down everything could change. Remember, You are trying to eat up most of the negative equity on your trade-in.
The bak will also look at your gross income per month and your credit score.
If you have any other concerns contact me at: rjfont16@yahoo.com
References :

Lenders are not allowing large L.T.V. (loan to value) loans right now so anything you can find for $3,000.00 to $5,000.00 will never book high enough to borrow $10,000.00 against it.

You also have to consider your most likely at least 5-6 thousand dollars upside down on your existing loan.

Good luck.
References :
Finance Manager for a car dealer for over 9-years.