Where are the spark plugs located on the 2003 gmc envoy?

They are located along the top of the motor cenerline. You must remove the plastic resonator that says Vortec 4200. That will expose the coil packs that have electrical connections to them. Remove the bolts and lift the coil pack to the side to reveal the plug. Remove as you would any plug.

Replace with AC Delco iridium.

The coilpack for the rear cylinder is under the cowl and takes a long reach but a long extension will fit just right for removing the plug. The coilpack in the front has a plastic wire guard mounted over the access bolt. Forget about taking this wire guard off but do take the top off by popping the latches. This will give you enough room to get a socket on the bolt and when you get it loose, just turn the coilpack to your left at about 8 o'clock, and you should be able to pull it out.