Weirdness On Main Amplifier Power Feed

Hopefully, I post my dilemma in the correct subforum... My trunk stopped thumping after years of normal operation. At the amp, all fuses and 12v signal test good. However, the main power feed reads a steady 25.8mv--and not ~12v (+/-) as expected. I went up front and discovered a blown main fuse (which concerns me, obviously). I swapped in a new fuse then read 12v thru the fuse (on the amp side) and 12v at the amp. But the instant I turn on the headunit, the fuse fails. Turns out I've had the wrong fuse inline for years--the amp's manual specs 150A for the main and I've been using a 60A, so that's being corrected. Yet the amp worked fine until now. My two areas of concern/confusion, and my reasoon for posting here, are: 1) Why would a 60A fuse suffice for years for the main, yet suddenly fail the moment the headunit is powered on? 2) What is the significance of the 25.8mv on the along the cable when it is disconnected from the battery and amp? I suspect a short or worn cable jacket. I'm sort of stymied at the moment and I request some t'shooting guidance/suggestions.

With the help of suggestions found here: https://www.diymobileaudio.com/threads/weirdness-at-the-main-feed.469558/?post_id=6293061#post-6293061 I tracked the problem to a short caused by a poor choice in fuse holder mounting hardware.