i forgot how to make my fan to run at 7v

which wire should i switch...

put the black wire (neg.) to a -5Volt source

That or you could get a baybus that has the 7v low setting. I have all of my case fans on it.

yes baybusses are nice. rheostats work well also esp for high speed fans. it sounds like your just tring to get the wire trick though. so what you want to do is instead of pluging it to yellow and black you want to do yellow and red. yellow to the posative(red) and the red to the negative(black) and your all setup with a 7v fan.