Here is a picture of the 7.3 in my 1992 international. It is pretty much the same as the 6.9.
The red circles show where 2 of the glowplugs are located. And the blue shows where 3 of the injectors are located. 4 each on both sides, I only circle those that are visable in the picture.
There are a total of 8 glow plugs and 8 injectors. The glow plugs are next to each injectors. The glow plugs are wired in two banks left and rear but they both go through the same relay. I used to just check to see if they worked or not by unpluging each glow plug and using a test light that is hooked up to the positive side on the battery and touched the glow plug, if it lit up it was good. But then I'd also check the wire to make sure it would lite up when the relay was energized to make sure power was getting to the glow plugs.
Now since you say it will start with a shot of starting juice the injector pump should be good. I think too if the individual injectors are not spraying and do a streamline instead that can make it hard to start and will effect fuel economy.
On the newer electric setup on theses motors all this stuff is now located under the valve covers which is even more fun to play with. No not realy it is more of a pain when and injector goes south.[/img]