Probably a bad lamp out warning relay ($200 or so) or a loose/broken wire in the fuse box. Occasionally, the screw that holds the wires to the bottom of the fuse holder or relay plug will come loose -- the result is restricted current or low voltage to the lamp.

I simply replaced the wire from the fuse to the headlamp. I still have a lamp out warning bulb lit, but I have both low beams.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!