Here's my thought's on this.
When hunting for these imagine how the assembly line operated. The data processing section generated these for the build process, they were the instructions for the line workers. The setup persons would attach these by taping them on the parts so they could hum right along, many a time they would write a randon number on the sheet, the car and parts. I would assume that some workers could not follow printed instructions and found it a lot easier to just look at numbers. I have a 99 K2500 Suburban that has a 4 digit number writen all over the place, on front control arms, rear axle, tires, etc.
I think they did not put them there but rather left them there. Some would tear them off when they installed the component, maybe because they were too visable but others were to occupied to remove them and thus the reason you find them today. I would think the more options installed on the car the chances of finding more than one increases.
Yes, they still use them today but the dealers almost always take them out and toss them when cleaning the cars/trucks. I just one in a new truck than was unloaded off a car carrier last week.
I had ordered a new Olds 88 in 1984. When it came in the salesman called and said it's being unloaded as we speak. I went up to check it out and there were the broadcast sheets lying in the trunk attached to the wheel trims rings package. One thing I noticed was the word in large letters, "MARRIAGE" at the top. It meant the body was married to the chassis and this sheet was the instructions to finish the job. I took it and saved it. I asked the salesman and he said go ahead it only going in the trash when they clean it. I purchased a 1970 Chevelle SS454 new in March of 1970. I never saw a build sheet for the car untill about a year later. I had a fender bender and the local shop had to remove the right front fender to get out the wrinkle. When I went in to pick it up the owner (he had 70 Plymouth GTX 440 6pack and we raced a few times,he always won) gave me this sheet of paper (build sheet). He said when he removed the fender he found it taped to the inside. The car came from Kansas City (Leeds)!
[This message has been edited by elcamino (edited 09-19-99).]