Under the driver seat at that bundle of wires, it should be there. I know the rear signal/brake hots are there....so reverse should be there as well. It splits off that to the taillight harness.
About your third.....you're wanting to do it the hard way, even though it's not that hard. lol. You're making it harder on yourself than you have to. Seriously....snip the far passenger side wire on the third harness on the truck side, get a short chunk of wire crimped or soldered to it, then route it to the dome light, crimp or solder on a ring terminal. If I remember correctly, when sitting in the driver seat, the screw you want to mount the ring terminal to is the front, passenger side screw. Pull the light to confirm, but it's the black thing that is the positive. BAM! Done!
Basically you'll be doing this....
I'm going to do mine one of these days.