Short Circuit--but where?

[Nala7]

I quite stupidly created a short circuit by fooling around with a 12 volt lighter add-on, powered by a 12 volt lead off the under-dash fuse panel. I had forgotten to unhook the battery ground Now no power gets to anything--meaning nothing happens when I turn on the ignition switch, no engine check light, engine won't turn over, no headlights, no dash lights, no radio and so on. Dead as a doornail. There is charge from the battery, there is nice +12 volts from battery to engine ground. All fuses seem ok including the under hood 50 and 60 amp fuses. The 60amp under hood fuse is getting 12 volts and the 50 amp ones are showing continuity across terminals. A nasty short circuit--but I don't know where to begin to locate where it's open. The brake lights won't even go on so there is no power getiing to the under dash fuse box.

Did I blow out the main relay or the computer? Or did I melt down a wiring harness?

There are 3 terminals on the fuse box. When everything is working right, the left one gets 12 volts when key is on, the middle one I think is ground and the one on the right is always on. It was the "always on" terminal I had the lighter hooked up to. I had imagined that because these 3 terminals were on the fuse box that they would've been linked to fuses, but no fuses have blown.
How do I troubleshoot this problem to locate where the short is happening?

Let me know if you need more info.
Thanks in advance.