Thats the security alarm switch, If the car is locked by the owner using the remote and he unlocks it using the door key and not the remote a buzzer from the security alarm comes on, if you press this switch that alarm goes off.
Yes, you can fix the reverse camera in Maruti SuzukiSx4. However, it recommended that you must buy and install the camera from the Authorised Maruti Suzuki dealer. To track the nearest Maruti Suzuki dealer please visit the link below:
It depends on the usage and maintenance, we have seen that people having cars for more than a century and having cars that have done 16 lakh Kms. So, if you maintain your car and drive it safely, you can have it with you for a lifetime.