here is a reply to a daytime driving lights question on another site.
maybe it will give you a place to start.

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.