I agree with what's been recommended so far with using nice marine tinned copper wire and such. Another suggestion would be to make sure you have equal or larger gauge ground wires to match your power wires - your 1 ground cable to 2 power cables ratio is not a good one. Better to have more and bigger grounds than to not have enough. Too small of a ground path can cause lots of problems (excess heat (fire!), poor accessory function, etc.) - especially with a long run like yours on a roof rack.Started to troubleshoot an issue with my roofrack lights. There are 2 separate circuits and both were impacted so immediately suspected a bad earth.
The three 6AWG (2 separate +ve feeds and 1 -ve feed) are part of a harness I made that routes up through the snorkel to get to the roof.
This is what I found when I opened the harness.
View attachment 2252991
I can't remember ever seeing this level of corrosion. The 3 cables at this area had a cable shield and marine heat shrink with no external evidence of damage.
It's been up there less than a year. Not sure what would cause this level of electrical corrosion.