I first worked at a dealer doing diagnosis in 1989. There had been computers on cars since 1979 and hand held devices to read data were available but GM came out with a machine (CAMS) to do more. It had a small CRT screen atop a big metal cabinet that rolled around. When you hooked it up to read data it would prompt you to splice in diagnostic cables to the circuit in question. There was a huge cabinet full of these cables. It could then tell you which leg of the circuitry was suspect.
This was all something you could do with a DVOM in a couple minutes so the machine was never used except as a curiosity. Dealers were forced to buy them with all the cables and discs to update them. In the mid 90s they came out with a 32mb hand held device which was a good tool.
Back then (1989) all service bulletins were on paper. When a service bulletin was released a paper copy was sent to the dealer and it was (hopefully) filed correctly. There were thousands of these pages in a huge catalog like they used to use for parts. Researching a bulletin for a car was a daunting task and if the last user took it out and didn't put it back you were out of luck.
The other thing was technical support. It was so bad before computers we didn't call unless we were desperate. You could count on a 30 minute wait and then when they did answer, they had to search paper copies of everything. By the time electric cars came out the tech support was pretty good. Actually it was pretty good in the late 90s.