On your car there should be a red wire coming out of the back of the fuse box from the accessory fuse. If you follow that wire for about 16 inches or so, you'll see one of those big black splice thingies that Ford loved so dearly. Not counting the red wire, there should be 5 other wires coming out of that splice. One of those is black with a red stripe. That one goes through the firewall and is plugged into the neutral safety switch harness. Another black/red wire comes from the neutral safety switch harness, comes back through the firewall, returning to the underdash area, goes a pretty good ways and terminates at a clear plastic two-prong female connector. On the other prong there will be two wires going into it, but, the single wire at that connector is the black/red wire that then goes to the trunk area. Power goes from the accessory fuse to that splice thingy, through the firewall to the neutral safety switch, back through the firewall to the two prong connector, and then back to the lights.
Assuming once again that the fuse is getting power, and that it is not blown, so that power is actually leaving the fuse box along that red wire, my first guess at where the problem is would be that splice. The good news is that there is less than 3 feet of wire between the fuse box and the firewall connector, but, the bad news is that most of it is wrapped with a bunch of other stuff and you can't find it without unwrapping a little bit of the harness, and space is not exactly abundant in there.