is this what this circuit is actually for? pic attached

[JesseA422]

truck is a 77.
from what i understand this is a constant hot.
when i got this truck the orange wire for the interior lights was connected here. as there was not any interior lights properly wired up at the time i disconnected it, and use the top male as a fused constant 12v source for a radio. it worked fine for a year and now the fuse popped while just cruising down the road.
radio is now disconnected from connection.
when i replace the fuse, it pops.
when i disconnect the battery and replace the fuse, as soon as i reconnect the battery, the fuse pops. this is without anything connected to the top male connection.

any idea where the problem could be coming from? what do i need to trace to find the problem from here?