There isn't a power flower switch in Super Mario 64.
But if you're talking about the ds remake then, pick Mario.
On the sun platform (on the first room in the castle you encounter), look up when the light is already there (the light i'm talking about activates when you get 16 stars or something). If this was successful, then you should see a tower when you go down. Over there, a big red switch should be spotted. If you pressed the button, all the red blocks would appear.
NOTE: Don't ever ever open a red block if you don't have a hat, otherwise a bob-omb would instantly explode on you.
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Thats not a cap,but a power flower you can get it if you are wario,you can get power flower from ? Box and watch out for enemy in the ? Box
get the mario cap then use the flower power to float towards the screen from the bottom of the slide along the wall and you will get to a hidden area, then use another flower to float up or use wall kicks to get the star after hitting the switch
Unlock Wario and get the flower power in levels and you are metal.
hit a ? box or go to the mirror room
I don't know sorry I have the same problem.