my 03 malibu lights dim when i hit the brakes, but only for a secondI think it's because most recent GM vehicles use a sort of 'on demand' alternator setup. Basically, the alternator in older cars ALWAYS puts a load on the engine, producing current even when there's no need (battery's fully charged, no other real electrical loads). Newer vehicles somehow disengage (not to say no belt turning it, just sort of freewheeling the alternator) the alternators in these situations. So when the electrical load suddenly increases, the system is sometimes just a tick behind catching up, leading to a flicker of headlights, etc.
The 05 Terraza that my wife used to own was notorious for this, to the point that GM changed the design and replaced the alternator. You'd be driving down the road at night, and something or other cuts on (maybe the AC compressor), causing the headlights to briefly dim then jump back to normal brightness. Many owners reported having oncoming cars flash their lights back at them, as if they'd flashed their own. We were lucky enough that that didn't happen with us, and the problem was minimized greatly once the higher capacity alternator / reprogramming was done to ours.