There's a whole bunch of them on both sides of the engine block and on the back. You must remove whatever is in the way to get to the one you want. Since you didn't bother to state the engine size or say why you want to get to it, I have to assume one is corroded through and leaking or you want to replace one with a block-type engine heater. For most of us block heaters are not necessary, and are not the proper fix for a hard-starting problem. The instruction sheet will state the location of the core plug to remove to install the heater. Many GM engines don't have enough room to accommodate the extra depth needed for the heater.
If a core plug is leaking, it corroded through from the acids that normally build up in the coolant. We replace coolant every two years to get those acids out and to replenish its anti-corrosive and water pump lubricant additives that wear out. The other plugs will suffer the same fate soon so you may want to buy the entire kit from an auto parts store or engine machine shop. There's a kit made for each engine with the correct size and number of plugs.
Saturday, November 3rd, 2012 AT 7:11 PM