Why does the Door Warning light stay on in my Toyota Celica when all doors are closed?

Hi Queen Ki, you are correct to check the door jamb switches or as you put it the (on thing) which aside from turning on the courtesy lights also illuminate the dash mounted indicator. The boot or trunk as we know it here also has a door jamb switch which could be at fault. These are simple contact switches which close or complete a grounding circuit when you open the door or trunk. you may want to close the trunk repeatedly several time to see if you can get the faulty switch to reset or reseat properly. Once you have determined which one is at fault, get the shop to replace it knowing which one will shorten the repair time and cost. Otherwise they may have to remove each switch to test it and it will take more time...hope this helps...good luck