Short on Circuit 7 (not six) - 1974 Plymouth Valiant Scamp

I would start with disconnecting everything in that circuit... put in a fresh fuse and then go back and reconnecting things. This will not identify a problem with wiring leading to the item. It will only identify a problem with the item itself. Look inside bulb sockets such as the trunk light. Those sockets with springs in there sometimes get kinda jacked up over the years. Springs fall out of position... that little fiberboard with the contacts breaks and causes contacts to contact each other.

You mentioned repairs from a previous owner.... man, that's bad ju-ju! Double check his work! I had a 67 Galaxie fastback that kept blowing fuses for the dash lights. Put in a fuse, pull out the light switch, and POOF! Every time. Turned out, there was an unused light lead under the dash that the previous owner musta thought was a ground because there was no voltage on it (unless you turned on the lights). He wired that lead directly to the ground of an under-dash stereo he had installed. The stereo worked fine since it was bolted to the metal dash... so it was grounded. Unfortunately, so was that light lead and after I disconnected it, my dash lights stopped blowing fuses!