Okay....I have an 86 nissan 300zx..and i installed a deck and 4 speakers about a month ago and everything was fine..i later installed an amp and 2 10 inch subs about a week and a half ago..but now all of a sudden i start to drive my car and i pretty much lose all power with 15 minutes when i start with a full battery..now i unhooked the connection between the battery and the amp because i thought there was a short somewhere with the amps connection...so then i recharged my battery and it died again...so i am not sure if there is a short somewhere in my car's electrical....if my alternator is shot...if you think it has something to do with my deck even though i'm not using it..or if my battery's shot....i have no clue and any help would be extremely appreciated....thank you

 

I doubt it's the stereo doing it. Especially since you've disconnected the amp.

To test the battery, do this:
disconnect battery
slow charge overnight
test battery (with hand held voltmeter)
if battery is less than 12V, battery is dead replace it.
let battery sit for an half an hour or so
test battery (with hand held voltmeter)
if battery is less than 12V, battery is dead replace it.
if it still reads 12V after a few hours, your battery is fine.
reconnect battery
turn key to ON position
make sure battery light comes on
if not, alternator is probably dead. replace alternator
start car.
if alternator light comes on, alternator is dead. replace alternator.

pretty basic steps. hope they help.

...and from the ashes shall rise a new hope. And it shall be known as "Z". And it will be good.