Looks like I missed out on the fun!
The can with wires and 2 hoses is cruise control. It's not related to operating the engine, but if you follow the linkage on the bottom side you will find the IP. It will have the red/pink and green wires described below, as well as steel lines running to the cylinders and fuel input/overflow lines.
The glow plug relay that you have is not original (hence the missing spade lugs for that connector in your had). The glow plug controller on these trucks is a common failure point, and many folks opted to install a manual switch instead of repairing the controller. You may well have a controller on the other end of that disconnected 2-wire connector. Or you can keep using your switch.
These engines are quite cranky without glow plugs. Test each of yours with an ohmmeter (while disconnected from the wiring harness). Several types of plugs have been available for this engine, and some of the newer types are much nicer than the original AC 9G plugs that came with the engine. If your plugs are good you will have a low resistance value. AC 60G plugs run 0.7 Ohms. Whatever random plugs I have in my K10 are reading 2.5 Ohms. Burned-out plugs will register infinite (no connection). Lower resistance values mean more heat will be generated, which is a good thing. But Ohm's law tells us that a 0.7 Ohm plug @ 12 volts is drawing 17 amps. If the system didn't have physical limitations, 8 plugs would be drawing 137 amps. This is a huge draw, and it typically happens while you are trying to spin a starter motor (which is a bigger yet draw). This is why 6.2 trucks have dual batteries. You don't need both of them, but the starting process goes more smoothly if you can deliver more power.
Old plugs were rated for just a couple seconds at a time (and they burn out easily), and this shows up in the controller logic. Bosch Duraterm plugs are rated for continuous duty for hours at a time. So if you have new plugs the original controller will still be running them 2 (ish) seconds at a time, even though they can be run much more aggressively. This is one reason why I have kept the manual switch in my trucks. I can start much more quickly than I could with the old controller logic.
The starter motor should have a bracket at the front end attaching it to the block. Several issues can arise if this is missing, including shorter starter life and breaking the mounting holes off of the block.
So it's worth replacing if it is missing. I have come across 3 trucks (so far) that were missing brackets, so I bought one new from the dealership and used it as a pattern to make several more out of a piece of bed frame. They're nothing fancy, but I may have one if you are missing yours.
Also, this engine originally came with a direct-drive starter motor (AC MT-27). Later on GM replaced this with a gear-driven starter motor (AC MT-28). The MT-28 cranks noticeably faster than the older direct-drive style. When I bought mine I brought in the old style and was handed the new style along with a sheet telling me how they are interchangeable.
But, the newer one uses a shorter mounting bracket than the old one.
These engines have a few quirks, but overall, they are quite simple and very reliable. Once you have good glow plugs and no air in the lines, the engines are very easy to keep running. 12V at the IP (pink wire) is all that's required to start them. No electronics or complicated controllers to worry about. It's like having the simplicity of a carburetor with the dependability of fuel injection.
Lemme know what other questions you have. There are enough 6.2 guys on this site that we should be able to answer all your questions.
