amp and light problem in 2000 v6 stang

I just installed a amp and subs in my car everything goes well during the day time but at night my head lights are dimmer than usual and my system dont sound as good as it does during the day. when i turn one off the other works well...what could be the problem....this isn't my first system i had in the car

I would check the amp draw on your new unit. It appears you need a higher amp alternator or your present alternator isn't performing up to specifications.
You can buy an amp meter which you lay on the main wire and check if your alternator is going bad. This link shows a picture of one.
http://www.autopart.com/tools/toolsmain/tool/T_3014.htm