Define dim and flash. Do they totally turn off? Your alternator only puts out the amount of current/amps that is needed for the system. Usually there is not much of a load needed in the system so it just puts out a minimal amount of amps. A light is nothing more than a short so when you hit your brakes more amperage is needed and the alternator kicks in for this, but it takes a little time for it to kick in. What you see as a dimming of the headlights is the time between the alternator putting out its normal amperage at the level it is at and the a higher amperage needed when the brakes lights come on.
2000 Sahara, 3.5" Rubicon Express Lift, 1" spring spacer & 1" Body Lift, 35" BFG Mud tires, Front & Rear ARB, Rancho 9000 shocks, UCF Ultra High Belly Up