If what you are stating is correct, then no having one fuse blow does not make sense to take out one side. I mean it is definitely possible to design an amp with a fuse, that blew and caused this but it wouldn't make a lot of sense for that to be the design for the following reason.
At a minimum it should have an INPUT power fuse, so if each output channel were fused, it should have at least 3 fuses if not a separate one for each channel. If it only has two fuses, perhaps I am just not aware of what you already know, that it has an external fuse you aren't counting in the count of 2, but any aftermarket amp should have an input fuse if it has any fuses at all, then it wouldn't make sense to output fuse only side.
However, I don't understand this "super hard to get to". At most it seems like you'd have to take some screws out, which isn't "fun" per se but may end up being what you have to do.
First you should use a multimeter, with your source audio playing, and measure the input for that channel. It should be around 1V AC, maybe higher depending on your setup, rather than 0V AC. You can compare against the other input channel(s). If you're not getting audio input on that channel then either your source (head unit) has a problem or the wiring between the two has a problem.
Not knowing what its protection circuit is like, you could also unplug the wiring harness to the speakers on that side in case one has a short, measuring resistance between the wires and chassis ground (obviously one of each pair may be a ground but not the other), and measure the output voltage when it should have audio out, which should be single or doubt digit AC volts.
If there are potentiometers or other controls exclusive to the side that isn't playing, you might also spray some contact cleaner into them if an open type opposed to completely sealed, and work (rotate or slide) the pot or switch while the contact cleaner spray is still wet inside. You can also measure resistance between the switch contacts, and the pot wiper and both ends of the conductive/resistive trace it rides on.
If there is some other fault in the amp, you would look at what chips are present and get datasheets for any you can. You could trace the power input to each to make sure both channels have the same reading (since one works but the other doesn't), using mid-DC voltage range on a meter, and similarly you would trace the audio input and all along its path to the output using a similar multimeter range AC, or a lower range AC until the audio signal gets to the gain stage in the amp.
I might also ask my friend if s/he is sure the amp was working the last time it was tried, and if it has ever had a similar problem in the past.
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