I was an instructor at a post-secondary automotive repair vocational school. I used to tell my students "NEVER let a customer see their car with the dash removed. You will get blamed for everything that goes wrong with the car from that day forward, real or imagined. Mostly imagined."
Very few customers would have the understanding to know this is a pretty standard affair, so you are right, never let them see this. Heck, depending on the customer, let them see as little as possible.
The ashtray lid was broken on my '02 Aston Martin DB7 Vantage. Can you just remove a couple of screws and replace it? Of course not. Firstly, they are NLA. Secondly, the center console has to come out to remove the ashtray assembly. And by the way, there are no instructions for disassembly in the profusely illustrated service manual and the nearest dealer is 7 hours away. Right. Armed with a box of bandaids and a pitcher of gin and tonic, I dove in. Haven't had this much fun since my last root canal.
Bought a 2004 Dodge Ram pickup in "as is" condition. The car salesman showed me how everything worked including the heater. When I got it home I found out air only came out of the dash vents, and it would not select either the floor vents or the defroster vents.
With the aid of the shop manual, I was able to determine the plastic diverter door was broken inside the heater. I would have to remove the airbags, drop the steering wheel, remove the A pillar covers, remove the radio, remove the glovebox, drop the dashboard, then disassemble the heater.
When I started this job I discovered that someone had tried to do this without the aid of a service manual. I could see where they took a sawzall to structural steel that held up the dash. They also managed to cut some other wires by accident. When they tried to remove the airbag controller, they bent some pins on the connector, then plugged it in again causing a 12 volt short circuit the vaporized some pins. Apparently, they gave up and reassembled the mess.
I spent four full days on this job. Eventually I was able to get at the heater and replace the plastic part. I also was able to repair the airbag controller.
No wonder they sold this truck "as-is".
The 70 Coronet requires removal of the windshield to remove the dash. Having purchased a painted rolling shell with the front and rear glass previously installed, the dash completely gutted of its components, I did what real men do: Leave the new windshield in and lay on my back on the floorboards of the car for days on end rewiring it and reinstalling everything while looking at it upside down, doing much of the work by feel. Opting to go with the American Autowire Highway 15 wiring kit rather than a reproduction harness added to the challenge which included designing subharnesses by hand. I even kept the bulkhead connector! In the end it was well worth the effort and achy back.
I have had the dash out of my 70 and 71 Buick GS's more than a few times. Wiring is usually not the problem. Broken tabs on the dash plastic housing and the lack of good grounds cause most of the issues on these cars.
I've had the dash out of my 13 Lexus GS (Yeah, I like GS cars) to replace the clock. It is dead center of the dash and looks easy but it is pinned behind several other pieces that have to come out all the way down to the radio and the main driver cluster cover. I almost stuck a $12 clock in its place.
Before that, I had to replace nearly every light bulb in a 99 Tahoe and the fan/heater controls that had melted. The dash was so brittle that all the snap-tabs that held various things in place were breaking off. The 4x4 lights kept flickering after that even though I cleaned all the contacts (in the dash and at the transfer case) and straightened the pins.
A late-model Chevy 3/4 ton truck of a buddy's dad had a horrible smell inside. We started stripping stuff and finally narrowed it down to a dead mouse in the vents. The evaporator was full of nuts, seeds and fluff.
This is a photo taken after I started the reconstruction off my '74 Z28 -LT Camaro. I had just finished the sound deadening and installed the rear carpet when I thought to get a picture. On the first ride after completion, I found out the back of the heater box (out of site) had a hole punched in it where heated air escaped all the time. Luckily, I was able to cover it with aluminum duct tape, but I was not happy that I missed the opportunity to fix it when it could have been easy. There is no way I'm going back to that stage.
Had a '69 Sport Satellite that I had to rewire back in the 80s due to some VERY bad attempts at add-ons by previous owners so I feel your pain. I too did eveything on my back (that just doesn't sound good no matter what forum you're in), and swore I would never do it again. I recently just did some repairs to wiring on my stepdad's '71 Duster. Fornutaley, there was enough GOOD factory wiring that I was able to remove the bad stuff (again from bad attempts presumably made in the 70s by previous owners) that I was able to solder in good wire. Still no fun on your back, under the dash, in an A body. The fact that I am 6'1" and 230 lbs also doesn't help.
I once checked my sanity at the door when restoring an 81 TransAm. I doubled the mayhem by removing a good dash from the donor car in order to install it into the "good"one....and changing the firewall to an AC car at the same time. Slow and steady won the race even though I absolutely hate dealing with old, brittle electrics. Once I decided to take a lesson from the factory and build the whole dash off the car and install it it one piece my sanity was restored.