2003 Ford Escort Poor performance

Well when you have your lights on at night and on top of that turn on your defroster, it's normal for the illumination lights on your dash to dim a little becuase its discharging(which is normal) and uses a little more amps to power them and I know it does it when you put it in gear. If it dims a a lot then you probably have a battery or an alternator problem. But when you're idling and rev it up and the lights brighten, its nothing to worry about and also tells you your alternator is charging. With only 40K on your car its unlikely you alternator is getting ready to die. Go get your battery and alternator tested at your local autozone or advance auto store for free. I'm sure it will turn out fine.