I have a '99 Tahoe with a bizarre blinker/brake/tail light issue. From the beginning: Fast flash on the right blinker: replaced all bulbs on right rear side - no results. Replaced light assembly on rear right side & blinker bulbs (only) on left side. Worked perfectly for 3 blocks & problem began again only worse. Turn on the right blinker and the left blinker flashed. Turn on the left blinker and the right side flashed as well as the left side of the license plate. Turn on the head lights and all rear functioning lights become alarmingly dim. hit the breaks and the turn signal and rear hazard lights come on. I had recently used my trailer light connector. Years ago it was problematic so I removed it, taped off the ends & double checked grounds. Next: Replaced the flasher relay. All rear lights appeared to work as designed except fading ~badly~ when head lights were engaged. Tryied again and all worked perfectly and continued to work perfectly for about a month. Then, while driving at night, I braked & signaled...got a fast flash again. Only this time - I have no tail lights, brake lights or reverse lights but blinkers (dimly) work. Replaced flasher relay again...I now have brake lights but the flashers are wacked out again and if I turn on the headlights everything goes haywire again. replaced the mulit function switch. It lasted about 30 days and all problems started again, PLUS the horn stopped working. tested all wiring to and from lighting assemblies- all good. removed entire trailer light assembly and all power past that point is correct appears to be fine Have never blown a fuse replaced the head light switch (with a used one) and all problems were the same except when pressing the brake the reverse lights came on. Put the old one back in and back to all the other problems (sans reverse lights). In between getting the multi function switch replaced and the problem reoccuring, battery had to be replaced and at the same time new tips were put on the cables. And now you can add that the battery was disconnected for about 3 hours to let the computer reset and now I have no running lights, again, when the headlights are on. (big sigh) ground wire to rear lights, stripped, cleaned and returned and another ground added. Me, and two mechanics are at a complete loss. Any ideas? .