Be careful if you try to lower it manually.  The mechanism has a locked position that cannot be overcome by pushing down on the assembly.  Reach up under the bumper and look for the pivot arms. at the pivot point you put your fingers behind it and pull down.  This will move the arms past the lock position and allow the assembly to close.  I would suggest using a vacuum pump on the actuator to see if it is operating and holding vacuum.  If the seal is shot, you can replace the seal or the entire actuator.

Possibly either an acuator or a vacuum relay on the drivers side.  If you havent already been to corvette 101 give it a try, the troubleshooting guides are very good.