Why would you ever take $4500 for a car that's worth $14,500 on trade-in? Cash for Clunkers only makes since if the car is worth less than the money they are giving you for it, the $3500 or $4500, it doesn't mean that you won't owe on the car that you traded in. If you're upside down in your truck, you might as well hold onto it and get use out of it until you don't owe more than you can get out of it.
If you trade in your truck for Cash for Clunkers, you'll take $4500 for a truck that you owe $17,500 on. You'll get $4500 knocked off of a $13,000 car, meaning you'll owe $8500 on it and then the $17,500 you owe on you existing truck = $26,000 on a car that's only worth $13,000 brand new.

What I'm saying is, you can't get trade-in value and then get Cash for Clunkers too.