Pontiac Grand Prix security light flashing

Security light flashs for the standard GM alarm system for 10 mins every time I start the car. Car and alarm system operate normally except for flashing light. I had the tranmission rebuilt and they said they disconected the battery cable but are refusing to correct this problem they created. If the alarm every goes off the security light flashs for 10 mins to let you know someone tried to get into the car however there seams to be a code stuck in the unit as now it flash everytime I start the car. How do I reset the security system.