There is a little trick but it isn't hard.
Locate the slot in the inner metal part of the switch, line it up with the forward facing slot in the plastic part of the switch:
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when they are in line, gently push the tab on the metal rod into the slot - but just a bit at first. Leave it loose while you line up the tabs on the outer plastic part with their slots in the plastic casing of the switch:
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Next, get the slot (the one that kind of faces forward) in line with the tab/slot in the inner part of the switch:
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The slots that they engage are elongated so you can rotate the plastic casing a bit. Those are like that to allow the pulse wiper setting to be adjusted:
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Once the tangs on the plastic casing are in line with their slots, give another little push in - but don't pop it home yet. You should be looking like this:
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A final check that everything is lined up:
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and then pop it home with the palm of your hand:
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