That harness in the middle is shot bud.
The two big fat wires in there go to the glowplug relay. I can't remember what all other colors they used for the controller, I abandoned mine right away. Red with a green stripe is key on power, its shared between the injection pump and glow plug controller i believe.
If it were me, I would just start over on the glow plugs, you can buy or make a harness to go from a relay to the plugs, then you just need power to the relay, add the controller if you want to, or use a button on the dash to energize the relay temporarily. Bullet crimp connectors work well enough if you want the newer glowplugs, need to patch the old harness, or diy a new one. Smash them down if they are too loose. The older spade plugs I'm not as familiar with.
If you diy, 14 gauge to each plug from the relay is plenty, you could even share a ring terminal per pair. To get power TO the relay you want something a lot bigger, like a premade battery cable which is what I used because I had it laying around. 4 awg or bigger I think, but I dont recall if these plugs pull about 10 amps each or what.
