Having experience with the Chevy Ventures, to find the built-in child seat, it is usually in the seat behind the front passenger (at least on the van's I've been in) If I recall correctly, you pull up the headrest and can pull down the seat part. Keep in mind, this is basically just the strap portion of a carseat attached to the regular seat and there is a small seat cushion that flips down. My DD at 2 1/2 did not like riding in this seat and was much happier once she was able to ride in her regular carseat. It didn't seem to me that it would be very comfy if your little one tends to fall asleep in the car, as it doesn't have the sides like a regular carseat. The child also sits lower than if they were in a regular carseat since the cushion is only approx 2" think I think. Also, keep in mind this seat is only for kids who are old enough and weigh enough to be forward facing. The seats basically work just like a regular 5 pt harness carseat.
Off on another note, for those of you planning on using the backless boosters, please make sure your child is in the weight range for them. I know my DD's TurboBooster can be used for 30lbs on up with the back, but without the back it is 40lbs and up. So, a child who would be able to use it backless would weigh too much to use the built-in seats. (I think some of the backless ones may have 30lb min, but not sure)