Here are the instructions. You will need to follow them precisely to work.

Preparation
1. Close all doors, trunk, and hood.
2. The security system must be in 'disarm' mode. The key must be removed from the ignition key slot.

Enter Code-Learning Mode
3. Open the trunk and leave it open
4. Open the driver's door and sit in the driver's seat.
5. Close the driver's door.
6. Cycle the ignition switch five times between the 'OFF' position and position 2, (ignition 'ON', all dash warning lamps will illuminate). The red status LED will illuminate continuously, and the siren will 'chirp' once, to indicate that the code-learning mode has been initiated.

Do not start the engine.
The ignition switch cycling in step #6 must be preformed within 45 seconds.

Register/Delete ID Code(s)
7. Open the driver's door, (remain seated in driver's seat).
8. Close driver's door.
9. Press and release any button on the remote you wish to register into the system. The status LED will shut off momentarily to indicate that one ID code has been registered.
10. Repeat steps 7 through 9 to register the remaining three ID codes.

It is possible for the system to memorize a total of four different ID codes. As a new code is initialized into memory, the oldest code in memory is automatically deleted. If you had lost your remotes and wished to delete the lost remote ID codes from memory, you could initialize the ID code from a newly purchased replacement transmitter four times thereby deleting the previous ID codes from system memory.

Exiting Code-Learning mode
11. Open driver's door, and exit from vehicle, leaving the door open.
12. Close trunk.
13. Close driver's door. The LED will turn off and the siren will 'chirp' twice.
14. The initialization procedure is now completed; test all remotes to confirm operation

It could be a faulty module.