2008 CTS trunk won't unlock

The acutator in the latch assembly is not strong enough or is frozen you will most likley need to replace latch assembly to solve problem they are known to go bad on both the doors and the trunk and are a one piece unit with no serviceable parts

You hear relay clicking where on the vehicle ?
Rear Compartment Lid Release System Components
• Body control module (BCM)
• Remote control door lock receiver
• Interior rear compartment lid release switch
• Exterior rear compartment lid release switch
• Rear compartment lid release actuator
• Trunk release relay

Rear Compartment Lid Release Operation
The interior rear compartment lid release switch receives a signal from the driver door switch assembly (DDSA). When the interior rear compartment lid release switch is activated, the signal circuit is pulled to ground. The DDSA interprets this and sends a GMLAN serial data message to the body control module (BCM) requesting the rear compartment lid be released. The BCM responds by applying battery voltage to the trunk release relay control circuit which energizes the TRUNK RELES relay. The trunk release relay is supplied with battery voltage at all times. With the relay coil energized, the relay switch contacts close allowing battery voltage to flow to the rear compartment lid release actuator causing the rear compartment lid to release. The rear compartment lid release actuator is permanently grounded at G402.
The exterior rear compartment lid release switch receives a signal from the remote control door lock receiver (RCDLR). When the exterior rear compartment lid release switch is activated, the signal circuit is pulled to ground. The RCDLR interprets this and sends a GMLAN serial data message to the body control module (BCM) requesting the rear compartment lid be released. The BCM responds by applying battery voltage to the trunk release relay control circuit which energizes the TRUNK RELES relay. The trunk release relay is supplied with battery voltage at all times. With the relay coil energized, the relay switch contacts close allowing battery voltage to flow to the rear compartment lid release actuator causing the rear compartment lid to release. The exterior rear compartment lid release switch is permanently grounded at G401.

Trunk Release Inoperative !

Having a factory scan tool makes diagnosing this a lot easier .

Circuit/System Verification
Ignition ON, command the Trunk Release with a scan tool. The rear compartment lid latch should release the trunk lid.
? If the rear compartment lid latch is inoperative, refer to Rear Compartment Lid Malfunction.
Command the trunk release by using the interior trunk release switch. The rear compartment lid latch should release the trunk lid.
? If the rear compartment lid latch does not release, refer to Interior Trunk Release Switch Malfunction.
Command the trunk release by using the exterior rear compartment lid release switch. The rear compartment lid latch should release the trunk lid.
? If the rear compartment lid latch does not release, refer to Exterior Trunk Release Switch Malfunction.
Circuit/System Testing
Rear Compartment Lid Release Malfunction
Ignition OFF, disconnect the TRUNK/RELSE relay.
Test for less than 5 ohms between the ground circuit terminal 85 and ground.
? If greater than the specified range, test for an open/high resistance.
Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 30 and ground.
? If the test lamp illuminates, test the control circuit for a short to voltage.
Verify that a test lamp illuminates between the B+ circuit terminal 87 and ground.
? If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the TRUNK/RELSE fuse is open, test the control circuit terminal 30 for a short to ground.
Ignition OFF, disconnect the harness connector at the rear compartment lid latch.
Test for less than 1 ohm between the ground circuit terminal B and ground.
? If greater than the specified range, test the ground circuit for an open/high resistance.
Connect the harness connector at the rear compartment lid latch.
Ignition ON, connect a 15A fused jumper wire between the B+ circuit terminal 87 and the control circuit terminal 30. Verify the rear compartment lid latch is activated.
? If the rear compartment lid latch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the rear compartment lid latch.
Connect a test lamp between the control circuit terminal 86 and ground.
Command the trunk release ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
? If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.
? If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.
If all circuits test normal, test or replace the TRUNK/RELSE relay.

Hi Jeff,
I was going to say, check fuse/ jump the load side of relay Good post.