1967 Special/Skylark - This is the power wire for the harness. What does the other side of the firewall look like? mirror image? (The part above the firewall connectors)
The fuse block is a two piece unit. The part you see under the dash holds the fuses, the part behind it, is the (female terminals) connector side through the firewall that the two (male terminals) plug connectors mate that you see in your image.
That is the main buss connection from underhood engine harness to light and interior harness. Essentially, that large terminal connects to one side of EVERY fuse in the block.
