The rear bumper is more of a flexible urethane. This can be "glued" back together from the inside. There are several 2 part adhesives that are made to repair flex bumpers. The black(soot) will probably come off with a light polish. Then I would use wax paper and clamp the outside. usually use paint sticks. if you are careful, and just coat the inside of the bumper, you may be able to squeeze the crack to make it invisible. Then if it looks good, I put another coat of the bonding agent on the inside to build it up. Follow the instructions on the "glue" and it should last for some time.