2005 Chevy Silverado Gauges going crazy & interior ligh

I have a 05' silverado 2wd. Everything was working fine on it not a single problem. I was giving my brother a boost & cut off radio & all other eletrical devices. After boosting him off he took the cables off his battery & they touched together while still connected to my battery & instantly my radio came on by itself, fuel gauge had dropped to e causing low fuel light to come on. Later that day I discovered that when I hit the brakes the radio goes off & on, interior lights very dim, tail-lights not working, heater not working, gauges not working, gear indicator not lite but other cluster light are on, headlights are working. Both turn signals stay lite at the same time all the time. Seem like the more eletrical stuff I turn off the more any of the other stuff try to light up & work. Also when I hit the brake all the gauges jump in sync about five times then flatline. If radio up loud all gauges dance to beat constantly until I turn it down. It does all this stuff whether the switch is in the run position or the truck is started. All the battery cable are clean & tight. With a meter the battery registered 12.6, start the truck I climb up to about 13.7 so I, m guessing the alternator & battery are good but never took them off to have them tested. The starter turn over & crank the truck just fine with no drag at all. I haven't drove it to a repair shop or any place like that to get a code scan because the closeest place is about 7 to 8 miles away & I'm afraid the eletrical cooling fans might not be working properly & don't wanna take a chance of overheating my engine.