You will find a plug mounted under the dash on the drivers side. This is where the dignostic computer is hooked in to the car to see if any fault codes are showing on the ECM. It has 2 rows of pins. The top right hand side pin is pin A, the one right beside that is pin b. With the car off, put a paper clip or wire in both a and b pin to jump them. Turn the ignition to on but do not start the car. The service engine light will blink. The first blink is the first number and the second blink is the second number.
For example: The first number to blink should be 12 which means the ecm is working properly. The engine light will blink once, short pause, and then blink twice for 12.
A long pause will follow the second number and the same code will repeat twice more (each code shows three times). After code 12 shows 3 times then it will show the fault codes. After all the fault codes are shown it will blink 12 again.
Make sure to turn the ignition off before removing the jumper wire or clip from the plug or you risk damage to the ecm.
Take note of the fault codes and then look them up on the internet to get an idea of what you are dealing with. Here is a site with code listing. Scroll down for fault codes.
http://www.montecarloss.com/SSThunder/gmcodes.html
Hope this helps. Good luck.