This would normally mean that you are not charging. My guess is that things will eventually start to dim.
Check the battery voltage before starting the car, and then while running. The voltage should be higher while running. Somewhere greater than 13v. This is a primitive test, but it will give an indication of whether you are charging the battery.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)