Need some auto electrical help here please.

2003 Yukon. The 3rd brake light (one on top) is not working. I replaced the light fixture with a new one (old one quit working) checked the fuses and notta.

Can't find much on You tube and wondering what else to check/do?

It's not mandatory for State Inspection on this year model, but I would prefer to have the safety aspect of it shining when braking.

Any takers/ideas as what to check next??

I would leave it as it, especially if you are in AZ. As long as one tail lamp lights up when you apply pressure to the break pedal you are legal to be on the road, so if a Sheriff pulls you over for a broken tail light and arrests you for DUI you can get out of it. Ask me how I know?

voltmeter and check for voltage at the new light, check ground, try another bulb.

That might have been true in the past but not now; Case law is that the Federal standard can not be minimized by state statute on vehicle requirements. See SCHWINTZ v. STATE for one example.

Also, see the following regarding the new inspection regulations in Texas: New Inspection Regulations 2017

Its powered by the CHSML fuse in the left fuse block in the instrument panel. That fuse will not have power on it unless the brake pedal is pressed.

Because it's a GM product, check your parking lights under the headlights. They're famous for having one or both out. Seriously.

I have an older 90's Silverado that had a third brake light issue. The big tip off for me was when I accidently discovered it would work fine when the steering wheel was tilted up and then failed when the steering wheel was tilted down- I replaced the wiring harness in the steering column and resolved the issue.

FYI- Replacing that harness is pretty easy... except for a couple of plastic "blocks" that are part of the turn signal where it connects into the column to hold it in place. Just take a picture or study that assembly closely before diving in, it took me hours to figure out how to put it back correctly after the parts just fell out when I was disassembling it.