Nothing good comes of folks hacking at the factory wiring harness. Remember the good old aftermarket car alarms? How many headaches have those caused us over the years?(Wiring issues, parasidic drains, no-starts, broken wiring, re-wiring, and just anoying sirens.)
I still see folks hack there factory radio plug when for $20-30 you can buy a pigtail that plugs right in and then has wires to splice from. If needed a pre-fit plastic plate to hold the radio in the dash. Some even mark the pigtail leads with what it is. (IE - Power +, power -, memory power, peaker + left, speaker - left, etc.) You can wire up to the radio on the bench, walk over and plug into the factory connector. Way easier than trying to splice inside the dash.
It would be nice if the truck and car manufactures put a seperate fuse panel with 4-6 spots for relays for aftermarket stuff like radio's, chargers, brake controllers (They seem to be getting better on this one, sometimes just adding one at the factory.), after market guages, etc. Then at least you would know the prior owners did not need to hack the factory wiring and cause potential issues.(But some I guess would just do it anyhow.) Also leave a couple of holes in the firewall with rubber plugs we can use those to go through it.
