And mechanics with a ballpark figure on this
Or approx how many hours labor would take

TIA

Year? What's it doing?

.

why are you replacing the entire harness?

Horn started going off while the vehicle was parked was a continuous long honk not like the alarm… pulled the horn fuse out.
Then the hazards started going off and would not turn off had to disconnect the battery
Took it in and was told the harness was corroded due to water and needs be replaced.
And it's a 2018 F250

My old man bought a used 2022 f250 6.7L last month and after about a week it was dead, no crank, but dash lit up. I went over to check it out and popped the hood. There's a fuse box in the back of the engine bay that was missing the lid. Mechanics verified it was full of water. Replaced the fuse box, which I think was roughly $1,400. The truck kept having random electrical issues and sometimes wouldn't crank. Their next step was replacing the harness for $2,400K - $4,000K with no guarantee that would fix the problems. He elected to trade it into a Ford dealership (with full disclosure of the issues) instead for a significant loss. Got a brand new 2024 6.7L and totaled it less than 24 hours later.

In my research into the water issue, it's common for these trucks to get water in the fuse box even with the lid in place. There's definitely some drainage issues that are causing problems.