1999 GMC Yukon Daytime running lights dont come on and horn

I do not know about the horn problem, but did you check the fuses for the daytime running lights. If so and that was not the problem there are two other items you might want to check and varify are working.

The first is a diode connected to the lead from the wiring harness that sits on the right side of the steering column. If it tests fine then check item two.

Item two is a day time running light module that is a small circuit board that in mine sits in a closed free hanging black plastic box to the left of the steerinf cloumn, the bottom of the box opens and you will see the card, it pulls out. I do not know how to test the card since I just swapped mine out with another one to see if it was the problem. It was not my problem so I am still lookikng, most likley moving onto the trun signal high beam switch next. But your problem should be easier to correct then mine.

Good luck.