This is the second time that this has happened to me. the first time was around Sept.
I get in my car today to go to the chinese buffet (like we always do on thursdays) and my sub isn't working. After a while of fiddling with setting on my head unit i turn the car off and look at the amp. All of the wires were still in their places and connected tightly.
I looked at my fused distro block and both of the fuses (one for the sub amp and one for the component amp) and they both looked fine. I started the car again and still no bass.
I switched the fuses for the component and sub amp, turned the car on and I only got bass. So the problem is this fuse. I switch it out and everything is fine. So i take a closer look at the fuse ... it looks PERFECT. I take it inside and test it with an ohm meter ... it regesters. So i don't know why this fuse won't work in the distro block, but others will.
look ... it is intact:

Sounds like oxidation on the fuse holder contacts or the fuse, or a bad fuse holder or distru block. I'd replace the fuse holder (or the distru block if it's part of the same assembly) and see if it helps.
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the fuse dosent always blow where it is visible