Resistor for daytime running lights?

I'm creating a daytime running light with a relay. I know that doing it this way, I will get 100% headlight output, making it just the same as if the headlights were turned on. Can I somehow lower this output by using a resistor? If yes, then which resistor should I use? And is it safe with no side effects?

John, you are usually right on, but in this case I think the lights will be nowhere near half the output, much less 'bright' Headlamps decrease output at an exponential rate (~3.4), and brightness is the square of that. The image below is based on a 9006 headlamp, most US-Spec'd lamps are rated similiarly What was a popular way for the mfr at one time was to use the voltage drop across 2 diodes to decrease the voltage. A couple of bucks at rad-shack can get you a full wave bridge rectifier which can be configured to do this- just hook the dc+ to the 12V side and the headlamp to the dc- terminal. Dropping 1.5v will cut you down to ~50% brightness some source information: Daniel Stern Lighting