If the FCU will be at high level by doing it as FR suggests, then maybe it would be better to put the FCU in a more accessible place & run a feed from it to the flex outlet. But you needn't use 2.5mm². Match the outgoing cable size to the fuse in the FCU. In other words, if you have a 3A fuse, you don't need 2,5 - 1.0mm² would be more than sufficient.
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