I have a 99 GMC sierra ec 4x4. Im not sure where the pcm is located, im new to all of this. thanks in advance

 

If you're looking at the engine from the front of the truck, its just to the left of the battery. It's down in there a little ways, and there's a nifty plastic cover over it and some fantastic little clips holding it in. You'll need a 7mm socket to get the connectors off too.