Why don't the 'daytime driving lights' work on a 1999 GMC Suburban anymore?

Make sure your parking brake is all the way off. If you notice, whenever you step on the parking brake, it turns the driving lights OUT. Even one click on the parking brake may be causing them to not come on. Otherwise check your fuses and there is a relay for the running lights also. Check that as well. Check the DRL diode. It is located just left of center under the dash. Has one screw holding it to the cross bar; kind of a pain to get to, but fixed the problem.