Where is the brake relay on a 2000 tahoe

If all the lights have gone off it is most likely the fuse has blown. Try this website to show you where the fuse is most likely to be.

https://fuse-box.info/chevrolet/chevrolet-suburban-2000-2006-fuses-and-relay

Have you looked to see if the bulbs need to be changed , if you have it might be a blown fuse or the wiring could be messed up

It does sound like brake light switch, some have separate circuits for 3rd brake light. One question for ya... do you have trailer hitch/wiring on the car. Sometimes trailer wiring will short out and give same symptoms.

If the two rear brake light are working on each side, then fuse and relay are good, check the bulb.

have u ck fuses,,relay,,for power,,do u have a test light,,ck pig tail connection,,

No relation whatsoever. If the brake lights work but the high one does not - you have a bulb out. Fairly easy to replace. The sunroof would be a totally separate issue. How long has it not worked? Did it stop suddenly at the same time as anything else stopped working?

I had this problem, and to be honest, it happened to the left side about a week later...it's the board that the bulbs plug into...it's a very common problem with this model. So popular that you can normally get the socket board off the shelf at a parts store! it's extremely simple to replace, and I replaced all the bulbs at the same time!

YES -- process of elimination. Obviously there's a short circuit in the Brake/Headlight circuits - right? Blown fuse(s) is/are evident of this -- would you agree? Which fuse(s) is/are blowing? An analysis tells you that the only time a fuse blows is when the headlamps are turned on -- correct? Otherwise, brake lights work normally -- right? OK. Head lamps have relays which turn them on. Here's a wiring diagram of the 2000 Hyundai Sonata (NOT the Jeep Grand Cherokee)
It would appear the problem is with one or more bulbs or sockets or both. As you many notice, there are no connections between the headlamps and brake lamps except the parking lamps (which share the bulbs/sockets)
When the headlamps are turned on, so does the tail lamp relay. As you can see, these bulbs/sockets are 'DUAL' filament to ground (series-parallel circuit). Just a guess here - the short is in one of the lower rear brake/parking lamp bulbs/sockets.... just a guess.

Check for power out of brake light switch by brake pedal. Of its faulty , the 3rd brake light will still work. Hope this helps

the brake light switch. under dash behind brake peddle arm.