Here's what I have. I've had our '63 C60 in the shop doing some tinkering and wiring it. Today when I went to move it, it wouldn't turn over.

I jumped from the power supply to the trigger terminal and it started right up. I figured it would be an easy fix.
Before I start, I will say that the only wiring I have been doing is on the taillights - nothing more.
As I was poking around, I noticed that the ammeter is pegged on discharge whenever the key is on. I slapped my amp clamp on each battery cable and only have a .3 amp draw.
I unplugged the ignition switch and jumped from power (red) to start (purple). I get a good sized spark so there is obviously a draw, but nothing happens.

When I jump the wire, I can hear a sound almost like tuning fork or a vibrating spring that fades out in less than a second.
I put a remote starter switch on the starter solenoid and it turns it over just fine. The solenoid draws about 18 amps.
I unplugged the wiring harness at the firewall and jumped from power to start - and it turned over. Plugged it back in and had the same thing as before. Thinking I might have a short, I unplugged the harness at the firewall and jumped the power wires so I only had incoming power. The ammeter still pegged and I had no spark when I jumped from power to start at the ignition switch plug.
I need help.