I have a completely discharged (when I got it there was a paper clip between the posts) 120v capacitor. It is the same (physical) size as the capacitor my brother is currently using between his battery and amplifier in his car. I would like to know if I can charge this capacitor off my car battery and use it for my amplifier. I assume that it will only charge to the voltage I apply to it, but I would like to have an expert advise me on it before I try ( I'd just as soon not cook my amp). It says nothing about Farad capacity on it just the 120v designation. Thank you for any help you can provide.

First, a capacitor doesn't need to be charged first in order for it to function properly in some circuit. Forget trying to charge it across your car's battery.

Whether the one you have will work just like your brother's is an entirely different matter. Without knowing your cap's rating it's a case of by-guess-and-by-golly. It may even be open or shorted. You can check that if you've got a multimeter. Check for shorts with the ohms function; it should show a slowly increasing resistance as the cap charges.

I'd recommend checking the ratings of the capacitor that works in your brother's car, then buying a new one for yourself. They're not all that expensive.

I would like to know if I can charge this capacitor off my car battery and use it for my amplifier.
I assume you intend to power your amplifier from this capacitor, without any other power source other than the car battery.
It all depends on the current you will draw from the capacitor.
C x E = I x T
In your case C is unknown, but let's assume it's a standard 120 Volt type of 100 uF.
Let's set T to 1 second. And the minimum acceptable voltage for the amplifier to function at 9 Volt.
.
I = 100 X (12-9) : 1 = 300 mA for one second.
A very small amplifier may work on that for a few seconds.
So this is not a very good suggestion to power an amplifier!
Just use some dry cells!
;)