2008 Silverado rear bumper replacement w/sensors

Is the wiring already in place with all the full size trucks to accommodate backup sensors in the rear bumper? My original one is getting to the point that I'll have to replace it in the not too distant future and it doesn't have the sensors,but I thought about trying to find one with them.But before I spend the extra for a rear bumper with the backup sensors in it I'd like to know whether there's wiring in place to take advantage of them.