Agree with improved grounding locally.
I'll assume the engine is running and the battery is greater than 12VDC.
I would check the voltage at another fuse, say F10
if the problem is dirty switches removing the fuse and checking on the battery side of the the fuses (F1 F5 high and F2 F8 low) should then be the battery voltage.
for trouble shooting, You might want to consider making a heavy wire jumper (#6 / #4) direct from battery to the switch side of the fuses. to determine switches on supply of ground on return
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