The main reason to determine the fuse (and location) that powers the ceiling light closest to the front vent is that when you remove that fixture to see if in fact it has a splice feeding the 2 vent wires.....you don't accidently short the 12v and ground while checking things.
Better to remove the fuse first then open up and check if the vent wires feed from the light and if the splice connections are tight and good. This saves you having to replace the fuse if something shorts accidently during your examination.
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