How to Enter a CTS With a Dead Batteryby Leonardo R. Grabkowski
If your Cadillac CTS's battery is dead, you can simply use the ignition key to open the vehicle's door, rather than using the keyless entry remote. The keyless entry remote doesn't work when the battery is dead. However, not all vehicles have a standard door key. Some models come with the keyless access system. At first glance, it appears there is no way to get inside the CTS when the battery is dead. However, the device has a hidden door key.
Inspect the Cadillac CTS's key transmitter. Look right beneath the "Trunk Open" button. You'll see a thin button. Push this button.
Grasp the bottom end of the key transmitter. Pull the end downward. The device's door key will come out of the transmitter.
Insert the door key into the driver's side lock cylinder. Turn it to the left to unlock the door. Open the door using the handle.