Michael, there is no need to open source the software. Software doesn't suddenly go wrong, sure it might be shipped with bugs but all software is and the car worked when new so no change there.
What happens is the sensors or wires fail (and why self driving cars scare me as they will always fail, or have their operation impeded, ie. bird dropping on them, etc.). So regarding DIY servicing, which the manufacturer does not care about or even want as they want a new unit to be bought, you just need to replace the faulty sensor(s) or damaged wire (this was the problem with the VW Golf's automatic as there was an internal short in a wire).
There already is hundreds of programs you can download to work with modern cars fault detection software, and you can buy the plugs/leads to connect your computer to them. My friend has one for Alfa Romeos and my 156v6 has been plugged in and old error codes erased.
Anyway manufacturers have finally made the mechanical aspects of a car so reliable that they are very close to being treated like mere appliances that the often touted 'run until it fails and just get a new one when it does' concept is very close. IMO once we have electric cars this will be reality, especially as the electric motor takes a lot of the brake wear away.
Software DOES suddenly 'go wrong'. Look up the Therac 25. Software should be open source for consumer products, but, there do need to be safeguards. In the not too distant future, software will be responsible for an ever-increasing number of deaths worldwide. I spent a good portion of my career in medical device Class 3 with tons of software. You would not believe how pervasive software bugs are based on unanticipated 'random' events that when in the hands of a million users, become reality.
74 GTV with 10548's and Ingram pump
71 Spider 1750 BOMBER ; 1995 LS 78K tight fast car