On my GMC pickup I had the same problem when I turned on the lights while driving down the highway, and it was a burned fusible link. The link may not have burned away all the insulation around it, so it may be hard to see the break. However, the insulation over the fusible link will be extremely soft and you can probably just pull it apart.
On my truck there is a power junction mounted in the engine compartment on the firewall. The 10 gauge power lead from the starter runs up to the this junction, and from there it provides power to the fuse box, lights, etc. The Pontiac may have the power junction in a different place, but I bet the wiring scheme is similar. The fusible wire burned right where it attaches to this power junction.
You can buy a replacement GM fusible link from the Help! section of most auto parts stores. Its probably a 14 gauge fusible link embedded in a short 10 gauge wire. The one I bought had a ring terminal on one end and crimp connector on the other.
Bruce