Daytime running lights - 95 Century

The daytime running lights (DRL) on my 95 Century only work once in a while, hardly ever. Nighttime headlights work fine, either automatically with photocell or manually with switch. The wiring diagram indicates that the DRL is controlled by both a DRL module and a diode. Where is the DRL module located and how can I test it? Where is the diode located and how can I test it?