Well you have a lot of wires there so let's see if we can sort them out.
Starting with the large red wire and the smaller red wire. The large red wire is the main power input to the key switch. It is hot from the power junction in the engine bay where the alternator, Battery, and regulator meet. It provides power for all the switched accessories in the truck.
The smaller red wire I believe is the ignition wire for the coil and the power wire for the fuse panel that is controlled by the ignition switch.
You might be able to confirm this by checking the small red wire for power with the key off and then on. Look for IGN on the key switch where it connects. If so that would be connected to a pink wire. I haven't a clue where the white(gray) wire connects but if the smaller red wire is from the IGN, on the key, it might connect with the pink wire instead of the gray/white one. It looks white in the picture.
The purple wire goes to the purple wire and is for the starter solenoid. You should be able to see and ST on the key switch where it connects. It should run to the neutral start switch on the steering column.
The large brown wire with the smaller brown /white wire goes to the ACC on the key switch. They share that terminal so they can get power from the large battery wire without getting power from the IGN wire when the key is in the ACC position.
The large brown wire is the power feed wire for the fuse block flashers and wiper motor and others.
The small brown/white wire goes to the inside of the cab block behind the fuse panel and then connects to a brown wire on the engine side of the block which runs to the external voltage regulator for the alternator.
the two small brown wires are for the brake warning light (not emergency). One comes from the master cylinder proportioning valve to the key switch and connects with the other one which goes to the dash cluster plug to pin 2 in the cluster plug.
This wire runs to the key switch to allow the key switch to ground the wires when the key is in the start position to ground the warning bulb and turn on the light for a test to see if it is not blown. No positive is connected here, it only grounds the wires.
The two green wires that are dangling do the same thing to the temperature sender light wire to test it's bulb. If you have a gauge dash with a temperature gauge then you won't need these two wires.
That just leaves the gray/white wire? Can you tell what it is connected to on the key switch?
This diagram shows the key switch and all the wires that go to it. The arrows point to the brown wire and the brown/white striped wire.
Attachment 1641848
This one shows the Red wires teeing to the power loads that they supply and the pink wires teeing to the ignition coil and the fuse panel.
Attachment 1641849