Its not just the GM trucks....... its the cars too. All in the design of the steering column. Thieves would take a small hammer and knock a small hole in the side of the column, use the screwdriver to pry out the "gear" the ign. turns, then use the screwdriver to pry back the rod thats activates the actual switch. All done in seconds.
Not to say other makes can't be stolen too..... some you could break out the ign. and use a screwdriver to start them. Just that the thieves figured out the GMs were much easier than the other brands.
As was stated earlier..... a pro is gonna get whatever he wants. The joyriders or casual thiefs usually stole GM vehicles.
I don't agree that they were easier. They were all pretty easy back then, if you knew what to do.
We had plenty of Ford's and Chrysler's go through my shop, the ignition locks and switches were defeated in much the same manor, just in different places. There were some Imports in there too.
In this part of the country, it was really about selling the parts. GM's were the most popular cars, and they were stolen the most, for that reason.
For a while here, they could steal your car, strip it, leave it to be recovered by the police, buy it from the insurance co. at auction, for salvage, put it back together, and have it with a title for $1500. The state finally made it so you had to have a salvage license to buy them. Now the real "crooks" can't buy them, just the salvage ones.
I'm not saying the whole salvage business was crooked, but, when you bought a used front end, and got one, that was not even dirty, it did make you wonder.