If all the lights are not working at the same time, I would suspect a ground like already mentioned. But what wasn’t mentioned is where the ground is located. Remove the passenger’s side rear wheel liner (it’s not too difficult). The wiring harness for the rear lights runs from the junction block located by the battery. From there it goes under sail window and along the passenger’s side upper frame rail. All the wiring for the rear lights are in that harness as well as the cruise control, trunk light, and trunk blower. Once the wheel liner is removed you can see it. It’s been a while, but I think there are a couple grounds easily seen in the wheel area. One is for the trunk blower (blows air through those tubes on the forward trunk wall – one for the alternator, one for the coil) and is located almost right above the center of where the tire would be. It’s a pretty big ground and might be a ground for the lights too. There might be other grounds in the area.

Check to see if the trunk blower is working. It should come on with the radiator coolant fan. I think the PONTIAC lights and the trunk blower share the same ground.

I’m sorry man, but that was a hate comment that I made. I deleted it now.