2003 Buick LeSabre Limited daytime running lights/sensor

I have a 2003 Buick LeSabre Limited, V6, 105,000 Km. The daytime running lights do not operate. When I turn the car on all the outside lights go on and stay on whether it's light or dark outside. The headlight switch is new. Is the problem likely to be in the light sensor on the dash or in the daytime running lights relay? If it's the sensor, how easy is it to replace?
Thanks!