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I would go and study the truck wiring schematics and confirm at various points of the wiring to verify that it matches the schematics.
Looks like your on the right track with this, fixing PO's butchered hackery. I like soldering the wires, and sometimes do the easy soldering with those heat shrink solder crimps. Also, I like pull all connectors and cleaning those up and stuffing them with silicon dielectric grease or compound to prevent further corrosion. Not sure what one can do if the wire to crimp connector is bad, but I suppose one can cut and crimp in new connectors and then dielectric grease them. You could also use White lithium grease for extra protection from the elements. Makes a mess, but works.
So I spent a bit of time at it today and got my trusty test leads and started testing where I was getting power (12V or around that) and where I wasn't. Found I was getting 12V to some of the right places, so I sorted it so I was getting 12V to ALL the right places. Then I reran the grounds for both rear light clusters, and ran an additional ground wire from the driver's side down to the frame. Just doing this I've eliminated all the problems with the turn and running lights. Covered everything as everyone suggested, so should be good for some time to come!
I thought I was having more issues with the indicators, but then read online that with the electro-mechanical flasher units, you need a certain voltage for them to flash, so when I was trying it off the battery with all the lights on it didn't flash but when tried with engine running, I got it going.
The back up lights are next on my list to tackle, and I'm going to go about them in the exact same way I did with these lights...
Today my brake lights and turn signal lights quit working. I checked the fuse bottom left 20 Amp and it was blown so I replaced it and it seemed to do something except everything was going haywire like a ground issue even replaced the flasher relay but that didn't fix it. so I took off my taillight covers and low and behold the taillight assembly was only halfway plugged into the wire harness causing a short to yall pros this might be obvious but I'm just sharing my experience as a rookie