I have a 2000 GMC Sierra 2wd with the new body style. My hazards went crazy after I towed a rental trailer from Home Depot, then they stopped working all together. By crazy I mean they came on when the switch was off and wouldn't turn off until the battery died. Then they never worked again. The directionals still work and never had an issue. I bought the flasher unit and tried it on the one on the back of the fuse panel, but that just works the directionals, I thought maybe they were both on the same flasher. I can't locate another relay. I checked the panel to the left of the steering column and in the box under the hood and came up with nothing. I called the local dealership and the service guy couldn't get it to come up on the computer.
I'm going out of my mind. The reason it's such a big deal is the brake lights on my trailer aren't working anymore either. I've rewired the trailer, all new. I've rewired the truck with a new 7 way and even spliced the brake wire into the brake light instead of the factory wire for the plug and still get nothing. I even bought a new electric brake controller to replace the factory one. Everything seems to work except the trailer brake lights. (Yes I installed a converter for the two bulb system to a common bulb system) My last option is to fix the hazards and see if that also fixes the brake lights.
When I plug the trailer into my wifes '01 Suburban, the lights work fine, so I know the problem's with the truck.