when i am driving in the highway i think its past 80+ km/h i think it happens when im over 60km/h i have to recheck tonight. my driving lights and interior lights dim also my heater/a/c goes down. That happens when shift to neutral from the gear that i am in. When i go back to a speed ( 1,2,3,4,5,) everything goes back to normal.
i really dont want to get stuck in the winter its -40 out right now dont wanna freeze, can some one tell me whats going on with my 99 honda civic

forgot to mention i have aftermarket deck and jl amp and sub

 

easiest thing to try is check you alternator/regulator with a voltmeter. turn the car on with all accessories off, put the probes on the battery terminals. have someone rev up to 3000 rpm and hold it there. the voltage should be close to 14, and it should stay at one number, not cycling between numbers. Also check to see if the belt is tight.

If that was your problem it would be a nice easy diagnosis.

 

will try that thanks, hopefully thats it and nothing too bad. if this does not get fixed what could happen? would the car die?