In the middle (engine side) of the bulkhead connector there is a bolt (3/8" if I remember right). Back this bolt out maybe 20 rotations then pull both plugs forward (engine half of the harness and the lighting half of the harness) towards the front of the car and if it comes off then you can move on. If not loosen the bolt some more and then try unplugging it again.
Once the plug is apart you might have to clean out the gunk in it to see where the release tabs are at on the terminals and then you can take a small jewelers flat head screwdriver and work the locking tab back enough to release the terminal from the plug.
Once you get all of the wires out you want you may want to look at the other ones and see how they look. If they are corroded, clean them up and then apply some dielectric grease to the connector and put it back together being careful that the pins seat into the other half of the bulkhead connector and then tighten the center bolt.
I would then also apply some type of sealant back around the left over wires in the plug and into the holes where the other wires were at to keep any water from getting into the connector later on down the road.
If you do a search on my webshots.com albums I think I have a picture of the bulkhead connector (user name Custom_Jimmy or Custom_Jim).
Jim