How do you replace jsut the sliding part of rear truck window?

You have to buy the small panel of glass first if it's available, and then find a way to breake down the frame. That could be rivets,screws, or even a metal saw. but is usually more expensive than the whole slider itself. I am in auto glass and have only replaced a small panel once in 25 years. It is a pain and not worth the effor. Just buy a whole new unit.