It's in the far back under the overflow bottle.
In order to get to it you will need to remove the overflow bottle, which means you will need to drain about a gallon and a half of coolant. It's not easy removing the connector to the glow plug without breaking it. I have done it with about 18 inches of 16 guage wire. If you fold the wire in half and then take a pair of needle nose pliers and wind the folded end of the wire around the glow plug connector (2 wraps are about all you will get on the connector). After wrapping, twist the wire so it doesn't come off the connector. Then wrap the 2 (other) ends of the wires around a screw driver. Pull away from the head at an angle slightly down from horizontal. If you don't pull in the right direction you will snap the end of the connector. I used this technique to replace the cylinder 2 glow plug, which is by far easier to reach. I succeeded here and then proceeded to cylinder 4, at which point I snapped the connector. At this point I knew I was going to have to replace the entire glow plug cable so I just yanked 4, 6 and 8 off while I broke all the connectors.
Ford makes a tool for removing the glow plug connectors, which I bought and later tried to replace 1,3,5 and 7. Using the tool I snapped the glow plug connector on #1. I finished 1, 3 and 7. I spent so much time on 1,3 and 7 that I skipped 5. Later, I had some diagnostic work done to solve another symptom and decided to let Ford install the #5 glow plug. I gave them the cable which I had purchased and they installed it as well.
The reason I'm telling you this is to help you decide whether it's worth it to do it yourself. It's not a terrible job, but, it's not an easy one, either; and if you break the glow plug connector it will cost you $58 plus tax. So, in my opinion $350 is more than it's worth, however, it could be a pretty frustrating job if your not a contortionist.
Good luck....not trying to be a pessimist, just letting you know how to do it without a $70 tool and giving you a realistic view of the difficulty.
I did my own STC fitting, oil and EGR coolers and stand pipes and dummy plugs with success this summer, so, I'm not a rookie to do it yourself repairs and I did a botch job on the glow plugs. Just saying....
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