My 2004 Expy just started having an issue with lights dimming when I apply the brakes. It is only for a quick second. Could that be a faulty
brake light switch?

 

Which lights are dimming, dash, parking, headlights, controls such as heater, etc.?
Does it happen when you're moving, sitting at idle, engine on, engine off?
We need more information.

 

all lights (Dash, headlights, control, radio). It started doing that last night while driving. Haven't had a chance today to check the volts at the battery when it is off and when it is running.

 

If your battery tests good I would take a close look at each end of both battery cable. Check for corrosion and looseness.

 

Thanks, Done that. All is clean and tight. Battery had 12.96v (not running)
and 14.29 (running). Just seems a little weird that it would happen only when applying the brakes. I will check and make sure there is no wires grounding out in the area by the brake switch.
I will see what happens to night when I drive home from work.