Capacitor as passive crossover
 

Ok, want to use capacitors as passive crossovers for the speakers in my car. I want to hook the rears up to an amp aswell. I know the values I need and that they need t be bi-polar but I need to know what voltage to get. Each one would be getting 125 watts or 75 watts RMS. The speakers are 2-way 6.5" and are rated at 300 watts or 150 RMS and the little capacitor for the tweeter is rated at 50 volts so I'm thinking 50 volt should be fine but I dont want them to blow up. Like is there a way of finding out what kind of voltage it's pumping out, like a formula of some type? Any help would be awesome!

50v is plenty, ESPECIALLY for tweeters. Your amp is probably only putting out less than 40v to the 6.5's speakers anyhow, MUCH less to the tweets.

Easy way to check is to play music loud, and connect a voltmeter across the speakers terminals reading the AC voltage, and see what you get.

Put ur DMM on AC Voltage and measure the power comin out of your amp