Socata do not; they publish[ed] what they called WDs (working drawings) which were generic wiring diagrams, sometimes with an airframe serial number range, and if you had to do some wiring you have to trace the wires to see how it was done.
This is a right hassle for any avionics installer.
I wonder if Boeing keep individual wiring diagrams for every 737?
Probably naive of me to think that, but shouldn’t the wiring be the same in all aircraft of the same series? Why should there be “individual” diagrams?
An A&P friend and I went through the wiring on my then 40 year plane, cleaning and removing extra wiring for removed equipment, improving wiring quality for newer field installed equipment and so on. It was evident that the original harness had a lot of extra unused wiring for options not installed when the plane was new. Since none of those options will ever be installed now, we removed all that extra factory wiring and saved a little weight.
I don’t think the wiring diagrams would have been different for individual planes in my case, because the wiring harness used on all of them was evidently the same.
Mooney yes.
Beechcraft used to provide a customised avionics wiring diagram with each aircraft. Bonanza and Baron owners often still have the blue folders. There we no generic avionics wiring diagrams in the aircraft maintenance manuals. That changed when they went over to G1000 installations and all aircraft were built to a common standard,
Wiring diagrams on something like a 737 will be serial number specific. Also specific to an effectivity range, meaning effective to a range of serial numbers. They could also be modified for a particular customer. Then you have mod’s, STC’s, repairs, upgrades, ex-test aircraft. It can get difficult on an old airliner.
Older bizjets who’s operators have lost the completion wiring diagrams are always fun. Its really up to the operator/owner to keep hold of their wiring diagrams. My current employer does a good job of keeping the wiring standard across the fleet. It offers very limited customer option on standard production aircraft.
GA aircraft I’ve managed to use the manufactures wiring diagrams for most things. Permit/home build is always fun.
The other Q is whether the manufacturer passes the wiring diagrams to the dealer, and so on down the chain. For example Socata did AFAIK keep some sort of file on each S/N but these files never left the factory. And I am pretty sure they did not include the full wiring diagram.
Then the dealer, Air Touring in this case; went bust c. 2005, definitely kept a file on each S/N. I know this because their Receiver contacted me and asked if I want it. I said Yes and they posted in Special Delivery but with no stamps so I had to pay a fair bit 
There was some funny stuff in there, too…
The harnesses are standard on modern planes – they are made on a bench which is much faster – but that is just a generic one, without most options.
Then whether an avionics shop doing some work, years later, supplies the wiring to the owner, is another Q. I have known cases where they did not, and sometimes absolutely refused, saying their are the copyright of the shop. Also most owners probably don’t remember to ask for the whole work pack…
The importer/dealer for my plane committed suicide circa 1972 but the importer records floated around with one of the unsold new planes that was eventually moved on by his heirs. I ended up with electronic copies for the whole fleet and know for example the wholesale price and which ship the plane was transported on.
There’s a little individual airframe technical info, paint color codes, radios installed etc but not much.
I know legally in the UK all maintenance records had to kept on record by the maintenance company for two years. Not sure if this is still the case. I have know reason to think otherwise. So a copy of your wiring diagrams will at least be kept on file for a time. I cant see how a company could refuse to give you a copy of the wiring diagram for work they did on a aircraft you own.
I afraid it is down to the owner to make sure that they keep their aircrafts records up to date. But I think you are right. Most won’t know to ask. There are also a high percentage who don’t care.
Perhaps 3 issues:
I can write this stuff freely because, since 2005, I have been totally outside the EASA system, and been using trade services on an as-needed basis. And the only competent UK shop I have ever actually used has packed up GA work some years ago.
