Common problem with cars more than 10 years old - same problem that occurs with my audis airbag light effectively the connectors used to wire up the sensors are getting old. As a result dirt, dust and similar has got in there and is lowering the amount of surface that each of the connector is connected to on the other connector. Typically even if the connection is still there but the signal is lower the light will go on - so the best bet is to find which door is causing the problem and then locate the connector between that and the car computer - clean the connector and you may need to get the light reset. To aid this you can get a pc/laptop -> car connector from ebay and a program like VAGCOM - which will allow you to plug into your car and identify the problem/turn off lights