Transmitter programming per the GM manual:

Important:

Before entering the programming mode, verify the possession of all the transmitters you will be using on the vehicle. Once you program the transmitter, the memory erases the other transmitters. You may program up to 4 transmitters

1. Insert keys in ignition and leave in the OFF position.

2. Locate the OBDII Port under drivers side dashboard.
Note port is wider at top: Your OBDII Port will not have numbers, use this for reference.

3. Make sure there are metal contact pins in terminals 8 and 4. connect 8 and 4
with a suitable jumper wire (leave jumper wire connected throughout the
programming procedure.) Make sure you know which terminals are 8 and 4.
The vehicle will respond by locking and unlocking the doors to signify entering
of programming mode.

4. Close doors.
5. Turn ignition to the ON position ( as far as you can go without cranking the
engine ). Doors will lock.
6. Press and hold both the LOCK and UNLOCK buttons together on the first
remote to be programmed, up to 30 seconds, the vehicle will respond by
locking then unlocking the doors. Release buttons.
7. Within one minute, repeat step 6 for any additional remotes if any, including
existing remotes.
8. Remove jumper wire from OBDII Port.
9. Test Remote(*).