A Grass Valley woman is in serious condition after hitting another vehicle head-on Friday morning in Placer County; meanwhile, police are searching for a third driver believed to have caused the wreck.It was one of several wrecks in the area.Sandy Castle, 60, was listed in critical condition early Friday afternoon at Sutter Roseville Medical Center, but her condition was upgraded to serious later in the afternoon, hospital spokeswoman Liz Madison said.Castle was flown by REACH medical helicopter to the hospital; she had suffered major injuries, the California Highway Patrol reported.Police are searching for the driver of a white Acura Integra …