If you go into a dealer with a 12 year old used up clapped out old truck they will either "give" you about 3 times what it books at off retail on the new truck, OR they will tell you outright that they won't take it in on trade.
I've only ever bought two new vehicles and in both cases the dealer "gave" me way more on the trade in value than the vehicle was worth, but off RETAIL. They weren't fooling me, and I knew it penciled out to them giving me roughly book value on the trade off their best cash deal. I chose to suspend disbelief and let them think I thought they were giving me $5000 trade value on a $1800 truck.
With truck #3 pushing 12 years and 216,000 miles and booking at $1300 trade in value, I'm hoping to see the same treatment when I eventually start shopping.