Re: 2003 GMC Headlight Flicker
Isolator and Separator are two different things. The an isolator looks like a brick, separator looks like a small can and they function differently. Isolator might be in your engine compartment, separator is probably inside.
Either one can be disconnected to eliminate it as a problem. The separator also clicks, but I'm pretty sure a 2003 would have an isolator.
Also check your voltage on both sides, while running and not. The dash power ports should be on the van battery, and the ones in back on the house- one of those cheap plugin volt readers should tell you if the voltage in the system is fluctuating with the lights.