Bad grounds at GM computers

How often do you all find that the computer has bad grounds from corrosion on, let's say, 80's era cars? The computer is located up under the dash in most cars. It is a good place for it. How can the connections become corroded?

http://www.cardone.com/english/club/Products/ELECTRONICS/Protech/Tech_Bulletins/PT%2077-0008.pdf

Thanks.