Check closely right here:

 

I have 12v at all of these places until I put the bottom fuse back in, then it all goes away. So when that black wire with the orange stripe gets put into the circuit it causes a loss of 12v.

 

The orange and black wire goes to the stop light switch and won't cause a loss of voltage. The other side where I circled will. Check where that circular rivet is holding the clip for the side of the fuse with the red wire. I bet when you push the fuse in, you're putting pressure on that connection. To bypass if for troubleshooting, connect a jumper from the red wire around to the clip on the fuse holder on the other side.

 

Or, you can just short between the rivet I circled and the rivet above it on the next fuse up. If you now get your 12v that bottom connection is bad. You can see some rust under the rivet in the picture.

 

I haven't shorted this out yet, but I wanted to show you exactly what I am talking about. In the first picture I am connected to the back of the firewall connector and the fuse is pushed in on the side with the rivet you are talking about but not connected to the other side:





Then all I do is pivot it down until it touches the contacts on the other side and power goes away, I don't put any pressure on it to push it all the way in and the power is gone:

 

That fuse also feeds the courtesy lights. I assume you have the door open? When you have a corroded connection, you can read 12v until current flows, then the voltage is dropped by the resistance of the corrosion. The same thing can happen if you have a bad battery, it will read 12v until you put a load on it, then it drops. If I were troubleshooting at this point, I'd read the battery voltage, then turn the key to start and see what the voltage does. If it drops dramatically, you have a bad battery, bad connection on the battery, or a bad ground connection on the battery. Does the horn work? IF the battery checks good, I'd jump the 12v to the other side of that fuse and see if the 12v still drops. With the diagram and the information you have found, it's just a step by step check to find where you are losing it. I don't know what works or doesn't under the hood, but you said NOTHING works inside, is that correct? Based on what I'm hearing from you, it can be anywhere from the battery to the firewall connection. Put a charger or booster on the battery and see if it still happens. Run a jumper directly from the negative post to a chassis ground and see what changes.
Measure the 12v on the red wire of the firewall connector with it disconnected again and push the horn button. Does the voltage drop? The horn is independent of cabin voltages. Measure at the red wire on the horn relay on the radiator support and blow the horn, see if it drops there.

 

Well, it is working now I put a good charge on the battery and redid my previous voltage checks and the power stayed after putting the fuse back in. Then I tried some other loads like opening the headlight doors and turning on the lights and had no loss of power, so I attempted to start it and it fired right up. I then took if for a good drive and all seems to be well. My concern now is what actually happened? Did my shorting out the positive battery terminal cause the battery to be drained? It was just a momentary sizzle.

Thank you very much for all of the help.

 

great trouble shooting help from some very knowledgeble people----------interesting trace

 

It shouldn't be. I would still be leery of a bad connection somewhere that would show up again. A little driving would build up my confidence again. If I had any more related issues, I'd swap out a known good battery first, verify the connection on both posts and to the starter bolt and ground on the other ends of the cables. I hope you don't have any more trouble, but it makes me nervous when something starts working.

 

I am feeling nervous also, I definitely am going to get a new battery to take that out of the equation.