1998 Mercury Mystique lights go dim

the car starts and runs lately the battery light will come on occassionally, when the headlights are on they will dim, like they aren't getting enough power, last night on a longer trip they dimmed and did not come back to full strenght, then the engine seemed to lose all power, and it quit running, let it set about 40 minutes and it started and ran for about 5 miles then quit again, we called for a ride and left the car to pick it up today, what would we look at as the source of the problem