I've just learned that all GM radios beginning 1989, had a different rear connector and the antenna wire is not the same size either. However, it's quite simple to install a newer style GM radio in a pre-'89 car by using an adapter (GM part# 16140051) which includes the antenna adapter also. The rest is easy, as the radio itself will bolt in your stock GN dash opening without any modification.
Below is a picture of the adapter you'll need...
''49-blues'', when I asked my GM dealer for the adapter, I was told that altough the part number is correct, they cannot order the adapter, but that I could get it if I went to a GM-authorized repair center which has the exclusivity to diagnose and repair GM-only products. I don't remember the exact price, but it was around $20 -$25.