The tape deck was actually an upgrade option on 2005. Mine came with the CD/cassette. $18k for a 6 year old truck with that kind of mileage is pretty much top end retail value for this area.
If memory serves, it was a $100+ upgrade over the cd only so I'm not surprised that it isn't in very many. It's been worthless in mine ever since I took delivery of the truck and is sitting in a spare closet now.
ninja'd If I can remember, I'll look at the sticker for my truck. It's folded up and stuck in the case that contains the manual in the glovebox.