I got rear ended the other day and there is a fiberglass piece under the car between the rear bumper and gas tank that got completly cracked. its about 4 inches wide. Anyone know what this is or about how much it will be to replace?

 

Call the dealer?

 

The gas tank is nowhere near the bumper on a 4th Gen - just guessing on the year, since you didn't give any info about your car. Since you posted in LT1/LT4 Engine Tech, just guessing. Could you possibly mean the plastic vent in the side panel between the bumper cover and the gas fill tube?