Moving-On ; Going Forward : You sound like a Hands-On Person, which is where We-All started , so I'll make one relatively "short" final comment about "Repairing Modern Cars , and Their Wiring , BOTH "Hot-12" and "The Ground System"
When you first started working on Automotive Elec Systems, and Re-Connecting and Adding New "Stuff" you were probably handling wires that were a Lot Like what Ma-Bell used back in the 1950's for Wiring Phone Services in Buildings and Residences ! GONE ARE THOSE DAYS of decently SIZED , WORKABLE Wiring Revisions .... ! Todays Auto Wiring Mazes look a lot like what is also used on Modern Desk-Top PC , and LapTops ..... BUT THIS ANGEL-HAIR WIRING IS SCATTERED FROM BUMPER-TO-BUMPER , and everywhere in between ... ! It is absolutely "dangerous" (to the Vehicle) for Owner-Operators to be doing a lot of things that we used to routinely do 20-30-years ago in modifying OR REPAIRING Vehicles ; but even more so At The Dealeership , considering some of the People why consider themselves to be Auto-Mechanics !!!
What astounds me is that there is probably a lot more damaged wiring in our (used) vehicles that we know nothing about, until we start doing the NEXT JOB, and we AWAKEN the damage, and think that WE-JUST-SCREWED-UP .... when things go Side-Ways , or Plummet straight to .... !!!!!
Too much current is one of these itsy-bitsy-wires , all closely wrapped into pencil-thin multi-wire-harnesses , and the Whole-Assembly can have its Insulation melted into a consolidated Snake , but sstill "working" , .. then along comes an adventuresome , self-confident , and experienced newbee .... and POOF : one (or more likely several ) totally unrelated Systems all of a sudden are having Cardiac Symptoms !!!
The really SAD reality is , that the Same thing can happen to the Ground System !!!!!
And every manufacture , that used to Seal-Mig-Weld Unibody Sheet Metal Assemblies , now only Spot-Weld every couple FEET, with the Sheet-Metal Parts clamped on moving manufacturing Jigs ; then they (ALL !) apply (the Equivalent) HOT JB-WELD to all the inter-spacing "seams" , and by the time the assembly gets a few Yards farther down the line , this entire GLUED TOGETHER ASSEMBLY is used (cold) and ready to put the Skin on the Bones of a Skeleton that arrives from another Assembly Area ... ! To THIS "conducting" metal is added ELECTRONICS and WIRING HARNESES , ........... !!!
Then even a supposedly "trained" Mechanic tries to find a small hick-up !
Or a second or third owner tries to Bring Back To Life an unfortunate Victim of Reality-Land ... !
These Glued-Together Lego-Land Automobiles are suckering Good Owners , used to working on Quality, Properly Welded , Properly Framed , and Properly Wired Vehicles , into thinking that they can do the Act Of Creation, on a concoction that was headed into the Used Parts Bins from the Day is left the Factory ..... !
This is not going to end well for the Automobile Industry , and it may reach a crescendo in just a few more years : and for anyone who does not believ me ... Just Open Your Hood (if they still allow you to de even that !) and look at all the (combustible -!) plastic that only exists to keep the User of the Vehicle to NEVER SEE , let alone TOUCH any part of that SACRED COW !
FACT : most knowledgeable car operators could NOT jump start any recent model Vehicle if they were only several driving minutes away from a Bomb Shelter, and the SIRENS were blaring about an INCOMING THERMONUCLEAR MISSAL ! The Battery Negative GROUND is buried under several layers of Obstructing Plastic , none of which is Conductive for a Ground Path , and there is not a Metal Object of any kind Visible, except for the Metal Latch Assembly they touched while lifting up the Hood by Hand .... ! And so ends the Great Modern Automobile ! It works while it works , and it Dies an Eternal Death when it Dies for any "reason" !!!! Even the Dealers need to Re-Manufacture it in Whole Sections, b/c it was never designed , or built to be EASILY REPAIRED --- but it is supposed to Double or Quadruple the Dealers Profits AFTER Sale , by the very process of Re-Manufacturing , ... instead of REPAIRING what is actually mal-functioning ....