[Saudade]

Are you taking about the large connector mounted on the firewall by the brake booster? If so, I had a hard time splitting it as well on my '88. I had to keep rocking, and pulling. After a bit, I was able to get a small flat screwdriver in between to help wedge them apart. SOme of the terminals got corroded which was why it was stuck (and why some of my sensor reading were whacky).

If not this, then what part of the harness are you working on? Are you trying to pass it through a hole in the firewall? If you're going to replace it, you can simply cut the ends off.

As for why, that's hard to say without looking. Sounds like maybe you had a short somewhere in that area.