I haven't had one apart in a couple years (since I replaced one on a friend's MKIII) so the finer points escape me. But it does come apart and there are two screws which clamp the 100A ALT FL in place from below. Unbolt the fuse box, lift it up and take a look around. You may need to feed more wire from the wiring harness into the bottom of the box to create sufficient slack.
Nothing should require significant force.
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