I have a '91 350SDL...
When I turn on the headlamps (or just the running lights), The lamp-out indicator comes on. All of the lamps seem to be good(they all turn on). I am also having some other electrical probs with the car...fuel gague is not accurate but the reserve light works, the ASD light stays on and the ASD does not work. I have replaced the voltage regulator, which was bad, what's next?

Don't forget to check you rear window brake light.

Look closely at brightness of bulbs as the correct wattage is important. Many times the license tag lights are missed .

Thanks all ... I will take a closer look.