i'm not sure about trade-in but my SA call a 2nd hand car dealer to buy my car so that i can pay down payment with my old car

Make sure you know the second hand value of your car. Take a few quotes from elsewhere. I tried to trade in through my SA and they take my car for 3K less. In the end, I sold mine to another used car dealer.

yeap... on the same day, pass the car to them, and drive my forte home 
the 2nd hand car dealer will not pay me... they will pay direct to the NAZA dealer.
i told my SA b4 make order, i will only give u my car when i get my forte.
and i dun wan any hassle. if my current car(2003 wira aeroback with bad condition ) trade in value enuf to cover the remaining loan amount and for the downpayment, then only i'll get my forte from her.

and DANG, she settle everything, i just need to sign a few paper and get my forte 

I let a few used car dealer see my car then take a quotation. Tell them you can only pass them the car after you get your Forte. (price might differ a few hundred depending on when you get your Forte). Of cos they won't pay you yet , they will only pay you the very same day you pass them your old car.

If your Sa used car dealer give more or less the same price, then sell to the Sa dealer . Then you don't have to fork out downpayment. In my case , i don't mind paying for the d/p first as the price diff is 3k. I got my Forte, then the very next day, I sold off my old car. 

Hope that helps.