This is how to program your 1997, 1998, and 1999 (old body style) GMC Sierra Pickup Truck replacement remote keyless entry fob alarm by yourself at home without going to the dealer.

IMPORTANT: Before entering programming mode, have in possession all of the transmitters you will be using on the vehicle. Once you program the transmitter, all previous codes will be erased from the memory. The other transmitters will not be functional until you program the transmitters again.
1. Locate the Data Line Connector (DLC) underneath the driver’s side dashboard. Use the figure below as a map for your DLC (they usually aren’t numbered). The view in the picture below is looking straight into the cavities of the DLC. Notice how the DLC is tapered on both ends; we will be working on the wider row of cavities (1 through 8 in the figure below).
Use a jumper wire
to connect pins 4 & 8
2. Make sure there are metal contact pins in terminals 8 and 4 and connect the Data Line Connector (DLC) terminal 8 to terminal 4 with a suitable jumper wire (you can use a piece of thin electrical cable, speaker cable or simply U shaped paper clip; leave the jumper wire connected throughout the programming procedure). Make sure you know which terminals are 8 and 4. The vehicle will respond by cycling the locks (locking then unlocking doors) to signify entering of programming mode.
Figure a. Data link connector
3. Press and hold both the LOCK and UNLOCK buttons together on the first transmitter to be programmed. After about 15 seconds, the vehicle will respond by cycling the locks (locking then unlocking doors). Release the buttons on the transmitter.
4. Within one minute, repeat the previous step on the remaining transmitters to be programmed.
5. Remove jumper wire applied to the DLC (disconnect terminals 8 and 4).