My truck was in the Diesel Shop... the mechanic questioned "all the extra wiring under the dash".... Yes there was. I'd left the harness on the trailer brake full length as it had nice labels on each lead. I had installed an Escort Radar system (front & rear sensors wired to a under the dash box & a tiny control unit)... again left full length as they were tiny factory made connectors. They were all bundled & zip tied under the dash. None of these affect the engine but he didn't know what it was.
I had an AD150 & big line system installed.
Now, I'm adding a IAT fooler & XZT+. Both are plug-ins. This is important as they can be reversed for troubleshooting if necessary. I'm creating a schematic & sheet on the IAT. I AM going to copy & keep the installation instructions for the XZT. And I'm making a list of these aftermarket sustems... to stay with the vehicle. God forbid I should have to explain all the stuff on the truck... but better to keep it all together.
edit When I had the AC repaired last, the mechanics (idiots) apparently did not snip my zip ties... and some of my wiring (rear radar sensor) was damaged. It's my belief that they just used force. I'm thinking of employing grocery store velcro (produce dept green strips) to hold some slack in the wiring so it'll give this time.