Turning key to "run" causes all power loss.

Thanks for the help so far. Here's an update:

I performed the voltage drop test and I found an issue. With the key off, I got a 0 voltage drop from the battery to the alternator, which should cover the whole circuit. However, when I went to each of the components (relay, fusible link, bulkhead, ignition switch), they were all zero except for the fusible link connection, which registered about 0.001 to 0.006 VDC drop depending on how I moved the wiring around. I then moved the key to the run position and did the same thing. Battery to alternator was 0.6VDC, which I feel is probably high. I could hear the blower running, but then it stopped. Checking the battery to the ignition switch and my voltage drop was 11.99VDC (my battery was slightly drained through all of this). Walking my way down the wire from the battery, the relay was 0VDC drop, the battery-to-fusible link connection was warm to the touch, and jiggling it around caused the power to come back on (the blower started up again). I looked at the connection and the 8AWG wire inside the connector was holding on by a couple of strands of copper and it looked burned. The connector on the fusible link side looked a bit melted, too. I think I found my poor connection.

My intent is to run all new wiring from the relay to the interior.. I will also replace the fusible link with a 40A maxi blade fuse. And, while I'm at it, I'll pop a grommet through the firewall and solder/splice the power wiring into the interior and up to the ammeter. Or, it may be easier to just run the 8AWG wiring up to the ammeter and just abandon the old power wire. I'll use all new OFC 8AWG wiring and soldered connections. I assume that the bulkhead connector is likely burned and corroded based on the visible appearance. I tried to depin the bulkhead connector but was unsuccessful.